The purpose of the study was to examine whether coresidence with parents affects the reproductive success of daughters in modern Japanese society. In Study 1, I tested whether women who were living with parents at the time of marriage would experience earlier first childbirth. In Study 2, I tested whether women who were living with parents when their firstborn child was young would experience earlier second childbirth. Cox regression models were used to estimate the relative risk of reproduction, taking into account several potential confounders. The results indicated that parents-in-law, especially mothers-in-law (i.e., the husband's mother), exerted the strong positive effects on the reproduction of daughters. These findings imply that, in a traditionally patrilocal country such as Japan, the relationship between mother- and daughter-in-law influences reproductive success more than that between biological mother and daughter. It is thus necessary to consider cultural context when testing the impact of grandparental investment in modern society.
Possible effects of external and internal factors affecting prosociality in Japanese undergraduates were investigated. We employed social support as an external factor and helping norms, self-consciousness, other-consciousness, self-esteem, and religious attitude as internal factors. Prosociality toward friends/acquaintances was significantly positively correlated with social support from siblings, social support from friends/acquaintances, self-sacrifice norms, and private self-consciousness, whereas prosociality toward strangers was significantly positively correlated with social support from mothers, private self-consciousness, and self-esteem but negatively correlated with social support from siblings. The results support claims of an altruism niche that rest on the assumption that prosociality can be maintained only in an environment or a society in which altruistic acts are rewarded. Among the internal factors, private self-consciousness was the only factor found to correlate with both aspects of prosociality. Higher scores on private selfconsciousness were related to irrational altruism, making people less susceptible to the features of particular situations and, consequently, producing reputational benefits.