Masahito Morita

A comprehensive evolutionary account of human behavior demands simultaneous consideration of biology, local ecology, and culture. Historically, evolutionary anthropology integrated these dimensions through naturalistic observations which offered direct insight into how behaviors unfold within everyday settings. However, in many contemporary contexts, traditional observations are impractical due to challenges in accessing private spaces, contributing to the decline of such methods. While evolutionary anthropologists have gradually pivoted to experimentation and larger datasets, the contextual richness afforded by direct observation is not readily replaced by other approaches. In response, and in light of renewed interest in investigating mechanisms and methodological diversification, we introduce and reflect on autophotography —a visual method in which participants document aspects of their lives through photographs—as a pragmatic, quasi‑naturalistic observational tool for evolutionary anthropology. Within its limitations, this creative, participant‑led approach may help recover some strengths of naturalistic observation while adapting to contemporary field sites.

Adolescence is identified as a key period for identity development, but anthropological literature hints at cross-cultural variations. Past research suggests Japanese adolescents may experience more identity exploration/confusion compared to Western counterparts, but the reasons behind these findings remain unclear. This comparative study investigates how adolescents in England and Japan perceive and engage with their environments, and aims to identify similarities/differences that could explain cross-cultural variations in identity development. Using autophotography, 98 teenagers aged 13 to 15 years across 10 schools in England and Japan documented important aspects of their lives. We applied template analysis to qualitatively analyze these photographs/texts. Overall, we identified two shared themes between countries: “This Is Me,” reflecting personal identity through self-discovery and skill development, and “Connectedness,” emphasizing relationships and belonging. A Japan-specific theme, “Reflections Through Time,” captured the broader temporal lens of Japanese adolescents, sometimes with a sense of melancholy. This notably differed from English adolescents in our study, who focused on the present in a positive manner. These findings suggest cultural differences in how adolescents engage with their environments, which may contribute to cross-cultural variations in identity development. Our findings point to the importance of biocultural approaches in developmental research by considering the role of culture.

Various selection pressures have shaped human uniqueness, for instance, music. When and why did musical universality and diversity emerge? Our hypothesis is that “music” initially originated from manipulative calls with limited musical elements. Thereafter, vocalizations became more complex and flexible along with a greater degree of social learning. Finally, constructed musical instruments and the language faculty resulted in diverse and context‐specific music. Music precursors correspond to vocal communication among nonhuman primates, songbirds, and cetaceans. To place this scenario in hominin history, a three‐phase scheme for music evolution is presented herein. We emphasize (1) the evolution of sociality and life history in australopithecines, (2) the evolution of cognitive and learning abilities in early/middle Homo, and (3) cultural evolution, primarily in Homo sapiens. Human musical capacity and products should be due to the hominin‐specific combination of several biosocial features, including bipedalism, stable pair bonding, alloparenting, expanded brain size, and sexual selection.

In humans, support from partners and alloparents is crucial for successful child-rearing and optimal child development. However, the complex relationships among childcare support, children's outcomes and parental characteristics have not been fully examined. We investigate how three sources of partner and alloparental support—partner's childcare participation, support from children's grandparents and support from non-kin—can be associated with child social development. We hypothesize that the associations between childcare support from partners/alloparents and child social development are partly mediated by parental psychological condition and parenting style. To test this, we conducted path analyses on online survey data collected in 2016 from parents of 3- to 5-year-old children in Japan. We found no evidence that childcare support had direct positive effects on child social development. Rather, the benefit of childcare support was mediated by its effects on parental psychological condition and parenting style, which in turn improved children's outcomes. At the same time, we found some evidence that greater availability of childcare support was directly associated with more behavioural difficulties in children. Our findings reveal the complex pathways between childcare support, parental characteristics and children's outcomes in Japan, showing potential mechanisms behind parental and alloparental effects in industrialized populations. This article is part of the theme issue ‘Multidisciplinary perspectives on social support and maternal–child health’.

Recently, statistical analyses of demographic datasets have come to play an important role for studies into the evolution of human life history. In the first part of this paper, I highlight fertility decline, an evolutionarily paradoxical phenomenon in terms of fitness maximization. Then, I conduct a literature review regarding the effects of socioeconomic status on the number of offspring, especially in modern developed, (post‐)industrial, and low‐fertility societies. Although a non‐positive relationship between them has often been recognized as a general feature of fertility decline, there actually exists a great deal of variation. Based on the review, I discuss the association between socioeconomic success and reproductive success, and tackle an evolutionary question as to why people seek higher socioeconomic success that would not directly lead to higher reproductive success. It has been suggested that, in modern competitive environments, parents should set a higher value on their investment in children, and aim to have a smaller number of high‐quality children. Also, parents would maintain higher socioeconomic status for themselves so as to provide high‐levels of investment in their children. In the second part, I broadly consider seemingly evolutionarily (mal)adaptive outcomes besides fertility decline, including child abuse, menopause, and suicide. The integration of the major three approaches to human behavioral and psychological research (behavioral ecology, evolutionary psychology, and cultural evolution) could lead to a deeper understanding. I provide a model for the integrated approach. Rich empirical evidence accumulated in demographic studies, especially longitudinal and cross‐cultural resources, can assist to develop a theoretical framework.

Fertility decline is a great challenge to evolutionary approaches to human behavior. In this study, we apply the perspective of sexual conflict between mother and father to the fertility decline. We predict that, under serial monogamy allowing for mate changes, the ideal number of children for women should be smaller than that for men, because the cost of reproduction for women should be higher than that for men. Our reasoning is that if the cost of child-bearing and child-rearing is higher for women than men, and if women, who therefore would want a smaller number of children than their husbands, have gained more power in reproductive decision-making within a couple owing to the modernization of society, fertility should decline. Until now, few evolutionary studies have analyzed empirical data in modern developed societies with such a perspective. Our questionnaire survey in an urban area in Japan revealed that mothers did experience greater cost during childcare than fathers. However, in contrast to our prediction, we found no sex differences in the ideal number of children between mothers and their husbands in many cases. About 60% of parents remembered wanting two children when they were childless. Moreover, mothers and their husbands had equal power in their decision-making regarding having children. After presenting these results, we discuss some perspectives to advance our understanding of fertility decline in terms of sexual conflict.

Objectives To reveal the conditions that could facilitate childbirth in modern humans, it is necessary to analyze not only cross-sectional surveys but also panel data that track the same person for a long period. In this study, we analyzed factors that would influence the probability of childbirth. Methods We analyzed Japanese panel data with a Cox proportional hazard model. Subjects of our analysis were married women and their childbirth records from 2004 to 2009. Results Contrary to the predictions based on the theory of behavioral ecology, we found no positive relationship between good parental conditions for childcare, such as high income, increase in income, or coresidence with parents (i.e., grandparents of children), and the occurrence of childbirth. We found that the number of existing children had a significant impact on the probability of childbirth. The likelihood of further childbirth by couples with one child was nearly equal to that of childless ones. However, the corresponding likelihood of couples with two children was about five times lower than that of childless ones. Conclusions The total fertility rates in modern developed societies are quite low and couples prefer having two children. This trend is known as the two-child norm, but it is a paradoxical phenomenon in terms of fitness maximization. Our result provided new quantitative evidence of this norm. This study revealed that the number of existing children being less than two was one of the factors associated with further childbearing in our Japanese sample. Am. J. Hum. Biol. 28:220–225, 2016.