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 Social interaction between individuals has been deemed, in one way or another, to be a key factor underlying the evolutionary emergence of characteristically human traits, including extensive collaboration, prosociality, and cooperative communication. As a simplest form of social interaction, dyadic competition and cooperation are particularly well studied. However, when three or more individuals are involved, social interactions exhibit an additional facet of complexity due to coalition formation by a subset of the individuals, which may have a qualitatively different evolutionary consequence. Animal coalition as observed in some species of primates, carnivores, and cetaceans, is characterized by two or more individuals acting jointly against a third party in a competitive or aggressive context. Ihara (2024) developed a new mathematical model of three-player coalition game that is fairly simple, but still able to explain the observed diversity in animal coalitions. The model also supports the theoretical plausibility of the claim that coalition formation induced social selection favoring reduced aggression and lower fighting abilities during human evolution.
 Takuya Takahashi and Ayaka Onohara collaborated on cultural evolution of Japanese dialects (Takahashi, Onohara, and Ihara, 2023). Accent patterns of words play an essential role in the Japanese language. In the Tokyo dialect, for example, "ame" means "rain" when its first mora "a" is pronounced in a high pitch and the second mora "me" in a low pitch, whereas it means "candy" when the first and second morae are pronounced in low and high pitches, respectively. Each Japanese dialect is characterized by its pitch-accent system, defining the mapping between accent patterns and words. Studies have also identified accentual classes of words, or groups of words with the same accent patterns, that presumably existed in the common ancestor of the mainland Japanese dialects. In principle, the present-day dialects inherit these accentual classes, except that some of the distinctions between classes were lost, the phenomenon called accentual class merger, as the dialects diversified. Thus, the state of merger varies among dialects, constituting a useful source of information about shared ancestry. For a statistically based phylogenetic reconstruction, Takahashi et al. (2023) developed a new model to describe the evolution of pitch-accent systems, and integrated it into the framework of Bayesian phylogenetic inference with geographic diffusion (Takahashi and Ihara, 2023). Their analysis of data on pitch-accent systems of modern and historical Japanese dialects provided a statistical support for the monophyly of each of three conventional groups of dialects: Tokyo type, Keihan type, and N-kei type of Kyushu. They also suggested that the most recent common ancestor of Keihan-type dialects was located in or around the Kinki region and that of N-kei type was in northern/central Kyushu, while the result was ambiguous for Tokyo type.
 Yuri Nishikawa investigated the geographic variation of folk songs in the Ryukyu Archipelago, Japan, to infer the underlying process of cultural transmission (Nishikawa and Ihara, 2022). Cultural transmission between individuals is by no means a uniform process; in fact, traits are transmitted in different ways depending on their characteristics. A prime example is found in linguistic change, where words for basic concepts are known to be less liable to borrowing from neighboring languages than other words are. Regarding folk songs, it is plausible that songs sung in different social contexts are transmitted differently. To explore this possibility, Nishikawa analyzed published scores of 1,342 traditional songs, which had been collected from all over the Ryukyu Archipelago, using the CantoCore song classification scheme to quantify the musical distance (i.e., difference) between each pair of songs. The analysis suggested that horizontal transmission between islands/regions within the archipelago had played a major role in the formation of the current geographic patterns in the Ryukyuan songs. It was also indicated that the social context of the songs had affected how they were transmitted within and between populations; in particular, "work" songs exhibited a higher degree of between-island/region diversity, suggesting a smaller inclination to diversify within populations, than "child," "ritual," and "amusement" songs. In addition, the observed pattern of variation in the work songs among islands/regions were associated with the corresponding lexical variation, whereas no connection was detected between song variation and mitochondrial DNA variation.
 Takuya Takahashi developed a Markovian ancestral model to investigate the temporal and spatial dynamics of cultural evolution (Takahashi and Ihara, 2022). The model considers a network of populations connected by social pathways through which cultural diffusion occurs. Following the method of ancestral backward process, the model focuses on a cultural variant sampled in the present generation and tracks the lineage of the variant back in time to quantify the when and where of its original invention. Takahashi successfully derived analytical expressions for the equilibrium distributions of cultural age in each population and frequencies of variants originating from given populations. As a case study, the spatial pattern of dialect words within Japan was investigated (Takahashi and Ihara, 2020). It has been hypothesized that Japanese dialect variants exhibit a concentric geographic distribution centered at Kyoto, the old capital, and that this pattern can be explained by repeated inventions of new variants at Kyoto with subsequent outward diffusion. The supposed diffusion process was examined within the theoretical framework described above, using a model of population network in which new variants are created exclusively in a single cultural center. The equilibrium distribution of word ages obtained by the model replicated the observed pattern, suggesting that the emergence of concentric distributions is partially explicable by a center-periphery social structure.
 Masahito Morita and collaborators analyzed data from a 2016 survey on childcare support and its effects on parents' and children's outcomes in Japan to examine in what ways childcare support may affect children's social development (Morita et al., 2021). In humans, unlike other mammalian species, children are often cared for by not only their mother, but also their father, elder siblings, grandparents, and other relatives as well as non-kin, the phenomenon called alloparenting. Studies in small-scale, non-industrialized societies have found a positive association between child survival and the presence of kin, grandmothers and older siblings in particular, although there are other studies reporting null or negative associations. In industrialized populations, child mortality is generally low, and researchers are more interested in the effect of alloparenting on other aspects of child development, such as cognitive and language abilities, educational outcomes, and emotional stability. By means of path analysis, Morita showed that childcare support in Japan does not directly improve social development of children, but does have a positive effect on parents' psychological conditions and parenting style, which in turn positively affect children's outcomes. The finding indicated a potential mechanism behind childcare support, parental characteristics, and child development in industrialized societies.
 Saori Nojo investigated on the sexual selection hypothesis for human phenotypic diversity. The level of genetic differentiation among human populations is generally low as compared with those in great apes; nonetheless, there is a considerable amount of geographic variations in human phenotypes, such as skin pigmentation and craniofacial morphology. Furthermore, studies have pointed out that the extent of human inter-population phenotypic diversity is greater than what is explicable by genetic drift alone. The sexual selection hypothesis posits that the excess of phenotypic diversity is partially attributable to variations in preference for mating partners. Nojo explored this hypothesis using a computer simulation, in which the inter-population diversity of a quantitative trait is compared with selectively-neutral genetic loci (Nojo and Ihara, 2019). It was revealed that phenotypic diversification between populations connected by gene flow can be enhanced by people's preference for typical characteristics of their own populations or an arbitrary preference that is culturally transmitted within populations. As an empirical evaluation, an experiment was performed with participants from Okinawa Islanders and Mainland Japanese, a pair of neighboring populations, to study their preferences of faces (Nojo et al., 2022). It yielded partial support for the existence of such preferences as assumed in the simulation. These findings suggest that some of the human phenotypic variations may be understood as a product of cultural, rather than ecological, diversification.
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