Abstract
In humans, the allocation of resources, such as parental care and attention, is vital to offspring survival. Life history strategies are influenced by cues in the environment, particularly those that signal the availability of resources. What has yet to be determined is how individuals allocate resources to infants as a function of perceived ecological harshness and life history strategy. In the current research we hypothesized that perceived ecology would influence infant ratings (Study 1), and that visual attention to infant phenotypes would be associated with life history strategies (Study 2). Study 1 investigated the effect of ecological conditions (control vs. harsh) on preferences to infant phenotypes (i.e., underweight, average weight, overweight). Participants (N = 246) were less likely to rate infants favorably under a harsh ecological condition. Study 2 investigated visual perception in processing infant images. Using an eye-tracking task, participants (N = 239) viewed images of infants while their eye movements were recorded. Participants displayed an early attentional bias (i.e., first fixation duration) to the head of the infant and focused most of their visual attention to the torso of infants (i.e., total visit duration). The results of the both studies indicate that ecological factors play an important role in rating infants, and data from eye-tracking demonstrates that phenotypes influence the amount of attention given to infants.
Child survival in humans is contingent on the allocation of resources provided by parents and alloparents (Beauieu & Bugental, 2008; Burkart, Hrdy, & Van Schaik, 2009; Klug, Alonzo, & Bonsall, 2012). Directing care and investment towards children is likely to be affected by physical characteristics that signal viability. Characteristics of cuteness, youth, and health are motivating factors associated with parental investment (Golle, Probst, Mast, & Lobmaier, 2014; Volk & Atkinson, 2008, 2013). Human parents are more likely to be attentive to infants that are attractive, and parents of unattractive infants are more likely to be attentive to other people instead of their own infant, suggesting physical features influence the allocation of resources to parenting and/or mating (Langlois, Ritter, Casey, & Sawin, 1995; Stephan & Langlois, 1984). Attractive infants are likely to be viewed favorably, whereas infants low in attractiveness are viewed as causing problems to their parents (Schein & Langlois, 2015) and individuals have negative stereotypes about their abilities (Ritter, Casey, & Langlois, 1991). Additionally, infant health significantly impacts parental investment (Bugental & Happaney, 2004; Daly & Wilson, 1988). Mothers behave differently towards infants with physical abnormalities, such as vocalizing and imitating less compared to healthy infants (Langlois et al., 1995). Although research has demonstrated that cuteness elicits positive responses to infants, this has been shown to be mitigated if infants appear unhealthy (Golle et al., 2014).
Physical characteristics associated with body weight also play an inherent role in parental investment in infants. Body weight strongly correlates with infant survival within the first year (Mace, 2000), and care is likely to be higher for larger offspring (Klug et al., 2012) and offspring with greater survival prospects (Winkler, 1987). Humans peak adiposity during infancy and then decline to a leaner childhood period (Fomon, Haschke, Ziegler, & Nelson, 1982). During early postnatal months, newborns devote about 70% of growth expenditure to fat deposition (Kuzawa, 1998). As low body weight in infants can indicate lower chances of survival, parents may provide poorer parental care towards them (Franklin & Volk, 2018; Franklin, Volk, & Wong, 2018; Volk, Lukjanczuk, & Quinsey, 2005; Volk & Quinsey, 2006). Research has shown that infants and children with facial cues of low body weight are considered less cute and healthy, and adults desire less to provide parental care (i.e., adoption) to infants and children appearing underweight compared to those with healthy weight (Volk et al., 2005; Volk & Quinsey, 2006). Infant care can also be affected by psychological issues (i.e., anxiety, depression) that are associated with disturbances in attentional resources. Women with more attentional resources (e.g., low depression) are likely to care for at risk infants while care for at risk infants declines for women with limited resources (e.g., high depression) (Bealieu & Bugental, 2008; Bugental & Happaney, 2004).
Ecological influences and life history theory
Life history theory is a biological framework in explaining how organisms allocate resources over their lifetime (Kaplan & Gangestad, 2005; Stearns, 1992). Because energy is finite, there are trade-offs in allocating resources to the competing demands of survival and reproduction. In psychology, life history theory has been used to explain the individual differences that exists when making resource-allocation decisions, and these decisions constitutes one's life history strategy (Ellis, Figueredo, Brumbach, & Schlomer, 2009; Nettle & Frankenhius, 2020).
Differences in life history strategies within species are influenced by ecological factors, such as food supply and mortality hazards (Kaplan & Gangestad, 2005). Due to developmental plasticity, individuals can adapt to their local environment by shifting their life histories (Sear, 2020; West-Eberhard, 2003). Cues in the environment that signal an abundance, or a lack of resources, may result in different individual characteristics in strategies along a fast-slow continuum. For instance, faster life histories are characterized by having lower parental investment, earlier sexual maturation and reproduction, whereas individuals with a slower life history strategy are characterized by higher parental investment, later sexual maturation and delayed reproduction (Del Giudice, Gangestad, & Kaplan, 2015). Moreover, in reference to parental care, high extrinsic mortality risk may result in reduced parental investment, as investment cannot always reduce extrinsic risk (Del Giudice, 2009).
Experimental research using mortality cues has shown that perceived ecological factors, such as using harsh ecology primes, can influence decisions. In priming harsh ecologies, low SES women were more likely to show a desire for food and a reduced desire to lose weight (Hill, Rodeheffer, DelPriore, & Butterfield, 2013a). Further, ecological priming cues have shown to influence the preference for heavier set women. In looking at the ideal body size, both men and women from low SES who were primed with ecological cues preferred women who were heavier compared to the thin western standard (Hill, Rodeheffer, DelPriore, & Butterfield, 2013b). Harsh ecological priming has shown that individuals display favorable attitudes towards having children and marrying earlier (Stormer & Lummaa, 2014). Importantly, socioeconomic status can influence reproductive behavior. Individuals who felt poor during childhood were more likely to show favorable attitudes towards children and earlier marriage (Stormer & Lummaa, 2014). These findings point to the role of ecological priming impacting life history strategies, as harsh environments influence the allocation of resources to reproductive over somatic effort. However, no previous research has investigated the relationship between life history strategies on infant preferences, and what has yet to be determined is how individuals allocate cognitive resources to infants as a function of ecological cues, life history strategies, and infant phenotypes. We formulate this research question based on cognitive and behavioral traits that are associated with life history theory in psychology using a fast-slow continuum. Fast life history strategies are characterized by lower parental investment, while slow life history strategies are characterized by higher parental investment (Del Giudice et al., 2015).
Sex differences in infant preferences
Sex differences play an essential role in allocating resources to offspring. Parental investment theory suggests that care is influenced by the differences in energetic investment between the sexes, with females investing more than males due to their energetically expensive gametes (Klug et al., 2012; Trivers, 1972). Parental investment is also contingent on the availability of resources, where parents with more resources maybe able to invest in children with low viability compared to parents with low resources (Beauieu & Bugental, 2008).
Research investigating the perception of infants has found considerable sex differences in responses to infant faces. Women have been shown to be more sensitive to infant cuteness than men (Hahn, Xiao, Sprengelmeyer, & Perrett, 2013; Lobmaier, Sprengelmeyer, Wiffen, & Perret, 2010; Sprengelmeyer et al., 2009), suggesting that cuteness elicits favorable responses in care. Infant cuteness has been associated with being more friendly, cheerful, and likeable (Karraker & Stern, 1990). Infants elicit more interest by women compared to men, and this is particularly stronger the younger they are (Maestripieri & Pelka, 2002), while men have shown to demonstrate less effort in wanting to view infant faces (Hahn et al., 2013). Moreover, images of babies have been shown to influence life history trade-offs, such as current vs. future reproduction. In investigating desired years to marriage, women exposed to smiling baby images were more likely to want to marry sooner and were more likely to have frequent and positive thoughts of children compared to men (Lord, Holland, & Hill, 2018).
Eye tracking in perceiving infants
Pertinent to the current study, investigating visual processing of infant body types from a life history framework has not been explored to our knowledge. If there are differences in the allocation of resources in mating-parenting tradeoffs, visual processing may be able to reflect those differences due to its role in demonstrating overall interest. Examining visual processing through eye-tracking has gained momentum because it provides a proxy to attentional processes that can be objectively recorded, and it can provide insight into specific regions of interest a person is viewing (Corbetta et al., 1998; Krupp, 2008). Other studies, using similar methods of attentional capture, have shown that infant faces elicit more attention compared to adult faces (Brosch, Sander, & Scherer, 2007), and compared to older children and adolescents (Thompson-Booth et al., 2014). Eye tracking has been used extensively in understanding the physical features that are considered important in mating related behaviors (Dixson, Grimshaw, Linklater, & Dixson, 2011, 2014).
Research using an evolutionary framework has primarily focused on sex differences in the visual processing of infant faces. In a study investigating sex differences in viewing infants in nulliparous participants, women were more likely to view infant faces regardless of the sex of the infant compared to men. Men displayed attentional biases to infant faces only when paired with an adult male face (Cárdenas, Harris, & Becker, 2013). Given that this research used nulliparous adults, this research suggests that humans should have cognitive systems (i.e., baby schema) attuned to infant care considering the importance that cooperative breeding played throughout human evolution (Hrdy, 1999). In another study, mothers were presented with images of unfamiliar vs. familiar children displaying different emotional expressions (i.e., fear, happy, sad). Mothers were more likely to view their own children longer and displayed an attentional bias towards their children when displaying a fearful expression (Vandevivere, van de Brande, Bosmans, Mueller, & Braet, 2016). This indicates, that among parents, the cognitive system is attuned to identify one's own child and be attentive to emotional signals.
Current research
The current research investigates ecological factors and individual differences in life history strategies in attention to infant phenotypes. To date, there are no known studies that have incorporated a life history perspective on allocating cognitive resources to infants with different physical characteristics in body weight (e.g., underweight, average weight, overweight). By considering ecological and life history factors, predictions can be made on how individuals will allocate resources to infants given the harshness of the environment, and if individual differences in life history is associated with the allocation of resources. The current study investigates the role of ecological harshness on the perception of infant phenotypes (Study 1) and if specific phenotypes are attended to longer using an eye-tracking and rating preference task (Study 2).
Study 1
In Study 1 we predict that, 1) ecology would influence the perception of infant phenotypes, such that raters in the harsh condition would rate infants less favorably, 2) there should be relationships in individual differences in life history strategies (i.e., KSF-42) and rating infants favorably, where individuals on the faster end of the life history continuum would rate infants less favorably, while individuals on the slower end would rate infants more favorably, and 3) there would be sex differences in infant ratings, with women rating infants higher on all dependent variables. For Study 1, the dependent variables were ratings of attractiveness, health, viability, providing care, others providing care, and investment.
Study 2
Research using behavioral measures of attention, such as eye-tracking, has suggested that there are sex differences in viewing infants (Cárdenas et al., 2013). Because individuals must sample cues in the environment for decisions (Billington, Webster, Sherratt, Wilkie, & Hassall, 2020), using an eye-tracking paradigm may provide insight into specific physical characteristics or regions of interest that are important when viewing infants. Therefore, we investigated sex differences and individual differences in life history strategies in viewing infants with different phenotypes. In Study 2, we predicted the following: 1) Individual differences in life history strategies would be associated with viewing time to infant phenotypes with individuals on the slower life history continuum viewing infants longer. We make this prediction based on the high parental investment associated with ‘slow’ strategists (Del Giudice et al., 2015); and 2) there would be sex differences in viewing time to infant phenotypes, where it is predicted that women would view infants longer compared to men. For Study 2, the dependent variables were first fixation duration, total visit duration, and total dwell time.
Method
Participants
A power analysis to detect small effects (f = 0.10). revealed that 232 participants would be a suitable sample size. Participants were 246 undergraduate students from a Midwestern University, M = 19.54, SD = 1.89. Participant demographics consisted of 164 women (Mage = 19.26, SDage = 1.58) and 81 men (M = 20.10, SD = 2.30). They reported their ethnicities as Caucasian, 77%; African-American, 7%; Hispanic, 6%; Asian, 2%, and Native-American/Other, 8%. Two-hundred and forty-three participants indicated that they did not have children, and 3 participants did not disclose if they had children. One-hundred and sixteen participants reported being in a relationship and 130 reported being single.
Measures
Stimuli
Three Infant phenotypes images manipulated to represent an underweight, average weight, and overweight infant were used as the stimulus set (Neuberg & Krems, 2016, https://osf.io/5vtsq), see Fig. 1. The underweight infant differed from the average weight infant by making the rib cage more salient and displaying more fat in the arms, abdomen, and legs. The overweight infant differed from the average weight infant by including more fat in the neckline, chest, arms, abdomen, and legs.
Stimuli used for infant phenotypes. Left to right: Underweight, Average weight, Overweight
Citation: Culture and Evolution 20, 1; 10.1556/2055.2022.00019
Ecological manipulation
An ecological harshness manipulation was borrowed from Griskevicius, Dalton, Robertson, and Tybur (2011). Powerpoint slides were used to depict an ecologically harsh condition consisting of resource scarcity. The scenario of the manipulation reads, “The New Economics of the 21st Century: A Harsh and Unpredictable World”, and it presents images with information portraying a difficult economy, such as depicting unemployment rates and an uncertain economic future. For the control condition, participants were not given any information on the ecology. They simply rated the infant phenotypes across the dependent variables.
Life-history measure
The K-SF-42 life history instrument was used to measure individual differences in life history strategies. The K-SF-42 is a short form of the Arizona Life History Battery, and it is scored by summing up responses where higher scores are indicative of slow life history strategies and lower scores are indicative of fast life history strategies on a fast-slow continuum (r-k). Sample items include, “I contribute a great deal to the welfare and well-being of my blood relatives in the present”, and responses range from ‘−3 = strongly disagree’ to ‘+3 = strongly disagree’. The Cronbach's alpha indicated that the scale demonstrated good reliability (α = 0.88).
Perceived resource availability
Individual differences in resource availability was a 6-tiem scale from Griskevicius et al. (2011). The scale measures 6-items related to childhood socioeconomic status (3-items) and current socioeconomic status (3-items). Sample items include, “My family usually had enough money for things when I was growing up” (childhood), and “I have enough money to buy things I want” (current). Responses were on a 7-point Likert scale from “strongly disagree” to “strongly agree”. Although the scale can be broken down into two independent factors (child, adult), responses for the child items were highly correlated (r = 0.76) with the adult items, therefore we treated the scale as a global score on perceived resource availability.
Infant ratings
Infant ratings were measured by using a 7-point scale on the following characteristics: attractiveness, viability, health, likelihood of caring, likelihood that others would invest, and the likelihood that the participant would invest in the infant. Participants rated infants for their attractiveness (‘1 = unattractive’ to ‘7 = extremely attractive’), viability (‘1 = not viable’ to ‘7 = extremely viable’) and health (‘1 = not healthy’ to ‘7 = extremely healthy’). Responses for caring for the infant, likelihood that others would care, and the likelihood that participants would invest were scored on a ‘1 = not likely’ to ‘7 = extremely likely’ response scale.
Procedure
The study was announced on the university's SONA system, where participants were informed about a study on rating infant images. Upon obtaining consent, participants were asked to fill out a series of demographic questions on Qualtrics, such as age, sex, relationship status, number of children, and frequency of interacting with children. They were then asked to complete the KSF-42 life history scale (Figueredo et al., 2017). Once complete with the surveys, participants were randomly assigned to a control and harsh condition. In the harsh condition, participants were shown 7 PowerPoint slides depicting ecological harshness, such as a struggling economy and an uncertain future. Participants were shown one slide at a time, and they were required to click the next button to proceed to the following slide. Once complete with the harshness prime, they were shown three infant images in random order and asked to rate them on a 1-7 Likert scale on the following characteristics: attractiveness, viability, health, likelihood or providing care, likelihood of others investing, and likelihood of themselves investing in the infant. The rating procedure was displayed on the same screen as the infant image and were presented in a fixed order. The control condition was not shown any slides and were asked to rate the infants on the dependent variables.
Ethics
The institutional review board at Oklahoma State University approved this study (AS-19-7). All participants gave consent to participate.
Results
Descriptives
In this sample, participants were mostly nulliparous, where only one participant indicated that they had children. Participants were also asked if they had frequent interaction with children using a 5-point Likert scale, where 1 = never, 2 = infrequently, 3 = occasionally, 4 = often, and 5 = regularly. Twenty-one participants reported that they regularly interacted with children, 40 reported often, 63 reported occasionally, 76 reported infrequently, and 45 reported never.
Statistical analyses
Analyses of infant ratings (i.e., attractiveness, viability, health, providing care, others providing care, and investment) were entered into a 2 (Sex: male/female) by 2 (Ecology: control/harsh) by 3 (Phenotype: underweight/average weight, overweight) mixed ANCOVA for each dependent variable with life history strategies (i.e., KSF-42) and perceived resource availability as covariates. Follow-up pairwise comparisons were conducted using Bonferroni corrections. Significant interactions with the covariates (i.e., life history and perceived resource availability) were followed up with Pearson's correlations.
Attractiveness
There was a significant main effect for phenotype, F (2, 478) = 4.62, P = 0.01, ɳ2p = 0.02. The average weight infant was rated higher on attractiveness (M = 4.27, SE = 0.12) compared to the underweight (M = 3.37, SE = 0.11) and overweight infant (M = 3.02, SE = 0.12). There was a significant three-way interaction between phenotype, ecology, and sex, F (2, 478) = 4.52, P = 0.02, ɳ2p = 0.02. We decomposed this interaction by running two Ecology by Phenotype mixed ANCOVAs separate for men and women. The two-way ANCOVA was significant for men, F (2, 154) = 3.22, P = 0.04, ɳ2p = 04, but not women, F (2, 320) = 0.89, P = 0.40, ɳ2p = 006. For men, significant differences were only noted in the control condition, where men rated the average weight infant higher on attractiveness (M = 4.24, SE = 0.28) compared to the overweight (M = 2.43, SE = 0.24) and underweight infant (M = 3.02, SE = 0.24), see Fig. 2.
Ratings of attractiveness for infant phenotypes across ecology for men. Note: P < 0.05*, P < 0.01**, P < 0.001***
Citation: Culture and Evolution 20, 1; 10.1556/2055.2022.00019
Health
There was a significant main effect for phenotype, F (2, 472) = 7.49, P < 0.001, ɳ2p = 0.03. The average weight infant was rated healthier (M = 6.12, SE = 0.10) compared to the overweight (M = 3.77, SE = 0.12) and underweight infant (M = 3.58, SE = 0.11). There was a significant interaction between ecology and phenotype, F (2, 472) = 21.46, P < 0.001, ɳ2p = 08. The average weight infant was rated healthier in the control condition (M = 6.88, SE = 0.14) compared to the harsh condition (M = 5.36, SE = 0.14), see Fig. 3. There were no other significant main effects or interactions.
Mean health ratings for infant phenotypes by experimental condition. Note: P < 0.05*, P < 0.01**, P < 0.001***
Citation: Culture and Evolution 20, 1; 10.1556/2055.2022.00019
Viability
There was a significant main effect for phenotypes, F (2, 480) = 3.63, P = 0.01, ɳ2p = 0.01, where the average weight infant was rated as more viable (M = 5.48, SE = 0.08) compared to the overweight (M = 4.49, SE = 10) and underweight infant (M = 4.06, SE = 0.09). A significant main effect for sex and phenotype, F (2, 480) = 3.21, P = 0.04, ɳ2p = 0.01, revealed that females were more likely to consider the overweight infant more viable (M = 4.62, SE = 0.13) compared to the underweight infant (M = 4.03, SE = 0.12) see Fig. 4, whereas this difference was not seen in males (Overweight, M = 4.18, Underweight, M = 4.12). There were no other significant main effects or interactions.
Mean viability ratings for infant phenotypes by sex. Note: P < 0.05*, P < 0.01**, P < 0.001***
Citation: Culture and Evolution 20, 1; 10.1556/2055.2022.00019
Providing care
For the dependent variable providing care, there was a significant interaction between sex and infant phenotypes, F (2, 472) = 5.85, P = 0.003, ɳ2p = 0.02. Females were more likely to provide care to the average weight infant (M = 5.97, SE = 0.10) compared to the underweight (M = 5.40, SE = 0.12) and overweight infant (M = 5.55, SE = 0.12), whereas men were more likely to provide care to the average weight infant (M = 5.48, SE = 0.14) compared to the overweight infant (M = 4.94, SE = 0.11) but this difference was not significant compared to the underweight infant (M = 5.41, M = 0.17), see Fig. 5. With the perception that others would provide care, a significant main effect revealed that participants perceived the average weight infant as more likely to receive care from others (M = 5.73, SE = 0.08) compared to the underweight (M = 5.17, SE = 0.09) and overweight infant (M = 5.02, SE = 0.10). There were no other significant main effects or interactions.
Mean care ratings for infant phenotypes by sex. Note: P < 0.05*, P < 0.01**, P < 0.001***
Citation: Culture and Evolution 20, 1; 10.1556/2055.2022.00019
Investment
There was a significant interaction for ecology and phenotypes, F (2, 476) = 75.05, P < 0.001, ɳ2p = 0.24. Significant differences were revealed for the overweight infant phenotype, where participants were more likely to invest in them in the control condition (M = 7.61, SE = 0.16) compared to the harsh condition (M = 6.21, SE = 0.16), see Fig. 6. There was also a significant interaction between life history strategies and investment to infant phenotypes, F (2, 476) = 5.78, P = 003, ɳ2p = 0.02. We followed up this interaction by running Pearson's correlations between life history strategies and investment ratings to each infant phenotype. There were significant correlations between life history strategies and investment to the underweight (r = 0.20, P = 0.001) and overweight infant (r = 0.27, P < 0.001), but not the average weight infant (r = 0.12, P = 0.06). This suggests that individuals with more slow life history-oriented strategies were more likely to invest in the underweight and overweight infant phenotype.
Mean investment ratings for infant phenotypes by experiential condition. Note: P < 0.05*, P < 0.01**, P < 0.001***
Citation: Culture and Evolution 20, 1; 10.1556/2055.2022.00019
Discussion
The ecology provides individuals with cues to the availability of resources in the environment. From this information, individuals engage in behaviors that enhance fitness by investing in maximizing fertility or growth (Kaplan & Gangestad, 2005). In this study, we investigated the evolutionary significance of rating infants on a set of characteristics, and if perceived ecological factors contributed to those ratings. Additionally, we explored sex differences in ratings, as according to parental investment theory, differences in the amount of obligatory investment in offspring influences overall parental investment (Trivers, 1972).
The results of study 1 showed that infant ratings were influenced by ecology, sex of the rater, and infant phenotype. Ecological factors alone influenced infant ratings, where in a harsh ecological context, infants were considered less healthy and less likely to be invested in. This suggests that the perception of a harsh ecology may constrain the perception of availability of resources in young nulliparous adults; therefore, individuals may not be willing to invest in infants compared to less harsh environments. Anthropological research has shown that in harsh environments where infanticide may be considered, infants may be less invested in because it is beneficial to allocate resources to remaining children that display cues of good health (Hrdy, 1999). This was further qualified by the influence of infant phenotypes, as differences in fat distribution increased the likelihood of being rated favorably. This was especially apparent for the average weight infant phenotype when rating attractiveness, health, viability, and the likelihood that others would provide care. In a harsh ecological context, the average weight infant was considered less healthy, and the overweight infant was less likely to be invested in compared to the control condition. This is an interesting finding, as it may suggest differences in the perception of excess fat is influenced by environmental context. Perhaps, individuals may have the resources to provide for the overweight infant in a control setting compared to a harsh context, where excessive fat may require more resources (i.e., healthcare, dieting) from a parent in a resource constrained environment. There was some evidence suggesting that individual differences in life history strategies would be more likely to invest in the infant phenotype that was under- and overweight. Slow life history strategists may invest more in parental care, and they may be more likely to have the resources to invest in infants that are displaying features associated with malnutrition (i.e., underweight) and excessive body fat (i.e., overweight). Thus, the investment of parents may be able to mitigate the effects of high-risk children (Beauieu & Bugental, 2008; Bugental, Corpuz, & Samec., 2013).
Sex differences revealed that women were more likely to rate infants favorably for dependent variables attractiveness, viability, and likelihood of providing care. The sex differences in infant ratings could reflect the overall interest in children that women display compared to men (Cárdenas et al., 2013), in addition to sex differences in childcare. Across cultures, there are differences in the amount of care given to children, where men spend less time compared to women (Geary, 2000). This does not indicate that men do not contribute to offspring care, as compared to other mammals, human males engage in high levels of paternal investment, and offspring survival is often dependent on it (Bjorklund & Shackelford, 1999). Studies have shown that women are more sensitive to infant cues, such as cuteness, and this may have impacted the sex differences in ratings (Lobmaier et al., 2010; Sprengelmeyer et al., 2009). Sex differences in infant ratings could also reflect the role of cooperative breeding and allomaternal care considering individuals were rating infants that were not being considered as their own (Cárdenas et al., 2013; Hrdy, 1999). Furthermore, exposure to infant images may activate what is known as “baby schema”, which could serve as a motivational drive to care for and invest in children (Brosch et al., 2007; Glocker et al., 2009; Lorenz, 1943). The fact that nulliparous individuals demonstrated differences in rating infants may highlight the adaptive feature of infant caregiving, as humans likely evolved a cooperative breeding system where other members other than the mother cared for other children (Carter, Ahnert, Grossmann, Hrdy, & Lamb, 2005).
There were limitations to study 1. For instance, the images that were presented were outlines of infant phenotypes and not real images of infants. Although this may lack ecological validity, our main focus was to use images that depicted differences in phenotypes (i.e., fat distribution) without including any confounding variables, such as sex of the infant, overall cuteness, and facial expression, as they have been shown to affect ratings and viewing time (Golle et al., 2014; Hildebrandt & Fitzgerald, 1978). Additionally, most of our participants were nulliparous, and there may exist differences in the allocation of resources across ecological priming when compared to parents. Given the nature of our sample (i.e., university sample), these findings specifically relate to young nulliparous adults. Using a much wider sample comparing parents vs. nulliparous adults warrants future work. Lastly, it would be important to include some information in the control condition to control for potential arousal in the ecological manipulation, in addition to using multiple manipulations of harshness. Perhaps, comparing how individuals allocate decision rules in a safe vs. harsh ecology would be an interesting avenue worth exploring.
The findings from this study show that nulliparous young adults' ratings of infants are affected by ecological priming. Additionally, sex differences highlight the importance between the differences in men and women on rating infants on viability, health, and likelihood of providing care.
Study 2
Although Study 1 investigated more conscious efforts on how infants are perceived, the following study investigated direct perception of infant phenotypes using an eye tracker to record eye movements. We extend on Study 1 by investigating if infant phenotypes are visually salient, and if specific regions of interest in infant bodies (i.e., head, torso) are attended to longer than others. Additionally, we looked into sex differences and the role of life-history strategy in visual processing of infant bodies.
Participants
A power analysis to detect small effects (f = 0.10). revealed that 164 participants would be a suitable sample size, and we continued with sample recruitment until the end of the school semester. A total of 239 participants from a Midwestern University, Mage = 19.02, SDage = 1.43 participated in this in lab study. Participants consisted of 96 men and 143 women. They reported their ethnicities as Caucasian, 75%; African-American, 8%; Hispanic, 7%; Asian, 2%; and Native-American/Other, 7%. Two participants indicated that they had children, while 1 preferred not to answer. One-hundred and fifty-one participants reported being in a relationship and 89 reported being single.
Materials and procedure
The stimuli and life history measures (K-SF-42) were the same as in Study 1. For Study 2, the Cronbach's alpha indicated that the scale demonstrated good reliability (α = 0.85). For perceived resource availability, responses for the child items were correlated with the adult items (r = 0.63), therefore, we treated it as a global score on perceived resource availability.
Eye-tracking measures
Three eye-tracking measures were used to analyze visual movements to infant phenotypes. First fixation duration was used as an early-onset measure of attentional processing, and it was defined as the average time of the first fixation on a region of interest (ROI). First fixation duration is often used to indicate attention to salient items upon first view. Total visit duration was defined as the average amount of time spent viewing a ROI per trial. Total dwell time was defined as the total amount of time spent viewing each manipulated image (i.e., underweight, average weight, overweight). To prevent a central fixation bias, we considered fixations as the point in which the eye became still over a region of interest after 200 ms from the onset of stimulus presentation. All eye-tracking data were recorded in milliseconds. Regions of interest were created using the Tobii X2-60 studio creator to indicate the head and torso as regions of interest for specific eye-tracking metrics. For the head region, we used the interest area creator to highlight the top of the head to the bottom of the neck. For the torso region, we highlighted the rest of the body, see Fig. 7.
The circle (blue) represents the ROI created for the head region, and the remaining outline represents the ROI created for the torso region. The figure depicts the average weight infant as an example
Citation: Culture and Evolution 20, 1; 10.1556/2055.2022.00019
Procedure
The study was announced on the university's SONA system where participants were informed about a study where they were to rate infant images. Upon obtaining consent, participants were asked to fill out a series of demographic questions on Qualtrics, such as age, sex, relationship status, number of children, frequency of interacting with children, and were then asked to complete a life-history instrument (K-SF-42). Once complete, they were instructed to sit in front of the eye tracker (Tobii X2-60), where they viewed 3 images of infants while their eye movements were tracked. Before viewing the infant images, a 5-point calibration was conducted to ensure that eye movements could be accurately tracked. Once calibration was completed, they were given instructions as to the eye-tracking task. Participants were instructed to view images of infants carefully and press the ‘spacebar’ when ready to proceed to the following image. Before the presentation of each stimulus, participants were displayed with a fixation cross ‘X’ in the center of the screen for 500 ms. Images were presented sequentially and in random order.
Ethics
The institutional review board at Oklahoma State University approved this study (AS-19-7). All participants gave consent to participate.
Results
Descriptives
In this sample, participants were mostly nulliparous, where two participants indicated that they had children, while one participant preferred not to answer. In reporting interaction with children, 10 participants indicated that they regularly interacted with children, 41 reported often, 79 reported occasionally, 87 reported infrequently, and 23 reported never.
Statistical analyses
To explore the relationship between life history strategy and visual attention to infant phenotypes, Pearson's correlations were run. There were no significant correlations between one's life history strategy and the amount of visual attention displayed to the underweight (r = −0.05, P = 0.40), average weight (r = −0.10, P = 0.13), and overweight infant (r = −0.04, P = 0.13).
Eye-tracking analyses (i.e., first-fixation duration, total visit duration, total time) were entered into 2(Sex: male/female) by 2(Region of interest (ROI): head, torso) by 3(Phenotype: underweight, average weight, overweight) mixed ANOVA with participant sex as a between subjects factor, ROI and phenotype as a within-subjects factor. All pairwise comparisons were conducted using a Bonferroni correction.
First fixation duration
For first fixation duration, there was a significant main effect for ROI, F (1, 244) = 33.48, P < 0.001, ɳ2p = 0.12. The head region (M = 287.96, SE = 7.72) was the area of interest most visually salient upon first view compared to the torso (M = 222.44, SE = 7.72). The two-way interaction between ROI and body type was significant, F (2, 488) = 4.63, P = 0.01, ɳ2p = 0.02, where the head region had longer first fixation durations for the underweight infant (M = 307.47, SE = 13.03) compared to average weight infant (M = 250.53, SE = 13.03) but not significantly different than the overweight infant (M = 307.47, SE = 13.03). A two-way interaction between ROI and sex was significant, F (1, 486) = 5.25, P = 0.02, ɳ2p = 0.02. First fixation durations were longer for men viewing the head region (M = 323.36, SE = 11.80) than women (M = 252.57, SE = 11.80), see Fig. 8.
Mean first fixation duration (ms) for ROI by sex. Note: P < 0.05*, P < 0.01**, P < 0.001***
Citation: Culture and Evolution 20, 1; 10.1556/2055.2022.00019
Total visit duration
There was a significant main effect for ROI, F (1, 238) = 293.67, P < 0.001, ɳ2p = 0.55. The region of interest that was viewed the longest was the torso (M = 973.87, SE = 23.68) compared to the head region (M = 433.37, SE = 23.68). This was further qualified by a ROI by body type interaction, F (2, 476) = 12.96, P < 0.001, ɳ2p = 0.05. The overweight infant torso was viewed longer (M = 1,057.66, SE = 28.40) compared to the average weight infant torso (M = 910.23 SE = 28.40) but not significantly different compared to the underweight infant torso (M = 953.73, SE = 28.40). There was no significant difference in visual time comparing the underweight infant torso (M = 953.73, SE = 28.40) and average weight infant torso (M = 910.23, SE = 28.40), see Fig. 9.
Mean total visit duration for ROI by infant phenotype. Note: P < 0.05*, P < 0.01**, P < 0.001***
Citation: Culture and Evolution 20, 1; 10.1556/2055.2022.00019
Dwell time
There was a significant main effect for body type, F (2, 476) = 17.16, P < 0.001, ɳ2p = 0.07. Follow-up pairwise analysis indicated that the overweight infant were viewed longer (M = 3,223.58, SE = 111.60) compared to average weight infant (M = 2,663.53, SE = 111.60), but they were not viewed significantly longer than the underweight infant (M = 3,179.59, SE = 110.60). The underweight infant was viewed longer compared to the average infant, see Fig. 10. The interaction between participant sex and infant phenotype was not significant, F (2, 476) = 0.42, P = 0.65, ɳ2p = 0.00.
Mean total dwell time (ms) by infant phenotype. Note: P < 0.05*, P < 0.01**, P < 0.001***
Citation: Culture and Evolution 20, 1; 10.1556/2055.2022.00019
Discussion
Study 2 investigated the visual salience of infant phenotypes at the early (i.e., first fixation duration) and later stages (i.e., total visit duration, total dwell time) of visual processing. In addition, we considered the role of life history strategies in the visual processing of infants. By investigating eye-movements, one goal of study 2 was to explore if individual differences in life-history were associated with viewing time across infant phenotypes.
The results showed that the overweight infant was viewed longer in comparison to the average weight but not underweight infant. Overweight infants may be attended to longer because they signal the availability of resources from the environment or from bi-parental care. Children with limited resources are at a higher risk of mortality (Geary, 2000), and research from hunter-gather societies suggest that mortality rates are higher for father-absent children (Hill & Hurtado, 1996). In investigating early automatic visual processes, results from first fixation durations showed that the head region was the most salient region upon first view, and in later stages of processing, infant torsos were viewed longer, especially for the overweight infant. Though we predicted sex differences in viewing time, those sex differences were only apparent during the early stages of visual processing with first fixations. Men had longer first fixations to the head region than women, suggesting that the head region may be a salient region of interest when scanning infants. This may suggest that men are attentive to facial features because it may serve as a cue to facial resemblance in order to solve the adaptive problem of paternity uncertainty. Previous studies have found similar findings of men displaying attentional biases to facial features of children (Platek, Burch, Panyavin, Wasserman, & Gallup, 2002; Wu, Suyong, Sun, Liu, & Luo, 2013). Similar findings have been demonstrated by Cárdenas et al. (2013), where men looked longer and fixated more on infant faces, however, this was dependent on being paired with an adult male face. Alternatively, women have been shown to be more proficient in facial recognition and outperform men in recognizing faces of the same gender with limited exposure time (Hansen, Zaichkowsky, & de Jon, 2021; Rehnman & Herlitz, 2007), which may result in viewing then less.
One goal of study 2 was to explore if individual differences in life history strategies were associated with viewing time to infant phenotypes. Differences in life history strategies may suggest that certain individuals are sensitive to environmental conditions that maximize fitness (Kaplan & Gangestad, 2005). Therefore, individuals that vary along the fast-slow continuum of life history may display different viewing patterns to the physical cues of the infants displayed. The role of life history strategies, as measured by the K-SF-42, (i.e., fast –slow continuum) was not associated with viewing time to infant phenotypes. This may indicate that life history strategies are not important in the assessment of infant phenotypes. The findings should be considered exploratory since we used a psychometric approach in defining life history strategies. Psychometric approaches generally aim at capturing clusters of traits that are biologically influenced (Figueredo et al., 2017) and that represent behavioral traits that capture life history strategies (Sear, 2020). Nonetheless, it would be interesting to use other life history indicators, such as early vs. late pubertal development or socioeconomic status affecting visual processing to infants.
Study 2 focused on viewing time to infants presented sequentially. There are limitations to this approach, as attentional bias cannot be inferred between infants. Previous studies have focused on presenting paired infant faces, or infant faces paired with adult faces to infer attentional bias. The rationale for presenting images sequentially was to address if specific regions of interest (i.e., head, torso) are visually salient when assessing infants. Although the current study used nulliparous adults, future work may want to consider if infants are visually salient when introducing a competing motive, such as a mating condition or a competing stimulus (e.g., attractive adult). This may be able to address the importance of attending to infants given competing demands in the environment. In addition, the current study used the KSF-42 as a measure of life history, and it did not include an experimental manipulation of ecology as in study 1. It is possible that a harsh ecology may induce different assessment patterns to infant phenotypes. A harsh ecology may promote an aversive response of less viewing time due to the perception that resources are not available in attending and caring for children. Future work may want to consider this experimental manipulation. To add, it would be of interest to look at facial comparisons with different levels of body fat, as in this study most of the fat distribution was centered around the body. Perhaps comparing sequential images of the head followed by the body would elucidate differences that exist between different sized regions of interest in this study. Lastly, the use of a single line drawing for each infant phenotype limits our generalizability. Future studies could benefit from using an extensive collection of infant phenotypes or faces in order to appropriately generalize the findings.
General discussion
Ecological cues influence trade-offs in energy that individuals allocate resources to. An ecology that signals an availability of resources and lower predation may prompt individuals to expend energy on growth and somatic repair, while a harsher ecology may prompt a strategy for early development and reproduction (Kaplan & Gangestad, 2005). Given that cues in the environment influence trade-offs in allocating resources, we investigated the role of perceived ecology in rating infants on a set of characteristics (Study 1) and the role of life history in assessing infant phenotypes (Study 2).
Study 1 demonstrated that harsh ecological priming influenced the likelihood of rating infants favorably. In the harsh ecology condition infants were less likely to be considered healthy and were less likely to be invested in compared to a control condition. In addition to the ecology, infant ratings were influenced by their phenotype. In a harsh ecology condition, the average weight infant was less likely to be rated healthier, and the overweight infant as less likely to be invested in compared to a control condition. There were also sex differences in infant ratings. Women, compared to men, were more likely to rate infants favorably, which is in line with previous research suggesting sex differences in attending to and caring for infants (Geary, 1998; Lobmaier et al., 2010; Sprengelmeyer et al., 2009; Trivers, 1972). According to parental investment theory, the obligatory investment in offspring by women compared to men influences parental investment (Trivers, 1972), even in nulliparous young adults. Overall, study 1 demonstrated that ecological cues, infant phenotypes and sex of the rater, are important factors in rating infant characteristics.
Study 2 investigated sex differences, phenotypes, and life history on overall viewing time to infants. Previous research has suggested that there are sex differences in viewing children (Cárdenas et al., 2013), however, the role of infant phenotypes and life history on viewing time have not been explored. The rationale for using a life history framework was to determine if psychological mechanisms are sensitive to environmental factors that can influence fitness (Kaplan & Gangestad, 2005). The findings in study 2 showed that men viewed the head region of infants longer than women in the early stages of processing, however, there were no sex differences in overall viewing time to specific regions. That is, overall, the torso was the area viewed the longest across trials. Additionally, when comparing across phenotypes, overweight infants were viewed longer compared to average weight infants suggesting that the increased fat distribution in infants is visually salient. Nonetheless, there were no significant relationships between life history strategies and viewing time to each infant phenotype. This may indicate that one's life history strategy is more significant in an effortful task, such as providing ratings, as opposed to a more automatic task, such as attention as measured by eye movements.
The current study adds to the existing literature on the role of ecology and life history on human behavior by demonstrating that individuals' ratings of children are influenced by ecological cues, even if those ecological cues are state-dependent compared to real world ecological harshness. More so, it adds a unique component to the literature on life history theory by utilizing individual differences in strategies in testing ratings and viewing time of infant phenotypes. Although life history strategy did not influence visual attention to infant phenotypes, perhaps future work may utilize actual measures of ecological harshness in assessing overall attention.
Both studies used infant silhouettes in rating and viewing infant phenotypes. Future work should consider actual images of infants to increase ecological validity. Silhouettes were used to reduce the influence that sex of an infant may have on making ratings and in viewing time, as our goal was to isolate the role of phenotype on influencing ratings and viewing time. However, we acknowledge that infant silhouettes that appear genderless may also impact ratings and visual processing, as individuals are not likely to come into contact with nude infants unless they are the primary caregiver. Furthermore, their genderless appearance may have influenced the amount of visual processing, since unusual images have been shown to attract attention (Jeck, Qin, Egeth, & Niebur, 2019). We hope to explore these issues in future work with actual infant images and a diverse sample instead of relying primarily on a university sample.
Conclusion
In summary, nulliparous adults demonstrate differences in rating infants as a function of perceived ecology, rater sex, and infant phenotype. Infant phenotypes are important physical features that individuals utilize in judging infant health, in addition to the likelihood of investing in infants. Furthermore, evidence from eye-tracking reveals that certain regions of infant bodies are more salient at the early stages of processing and overweight infants are viewed longer, hinting at the role of fat distribution in attracting visual attention in infants. The way adults process infant bodies is crucial in understanding the mechanisms involved in parental investment, and we hope to have provided a template for further research in this field.
Conflict of interest
The authors declare that there is no conflict of interest.
Data availability statement
The datasets generated during and/or analyzed during the current study are available from the corresponding author on reasonable request.
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