Lesley Newson

Why Are Modern Families Small? Toward an Evolutionary and Cultural Explanation for the Demographic Transition

As societies modernize, they go through what has become known as “the demographic transition;” couples begin to limit the size of their families. Models to explain this change assume that reproductive behavior is either under individual control or under social control. The evidence that social influence plays a role in the control of reproduction is strong, but the models cannot adequately explain why the development of small family norms always accompanies modernization. We suggest that the widening of social networks, which has been found to occur with modernization, is sufficient to explain the change in reproductive norms if it is assumed that (a) advice and comment on reproduction that passes among kin is more likely to encourage the creation of families than that which passes among nonkin and (b) this advice and comment influence the social norms induced from the communications. This would, through a process of cultural evolution, lead to the development of norms that make it increasingly difficult to have large families.

Individual consistency and flexibility in human social information use.

Copying others appears to be a cost-effective way of obtaining adaptive information, particularly when flexibly employed. However, adult humans differ considerably in their propensity to use information from others, even when this ‘social information’ is beneficial, raising the possibility that stable individual differences constrain flexibility in social information use. We used two dissimilar decision-making computer games to investigate whether individuals flexibly adjusted their use of social information to current conditions or whether they valued social information similarly in both games. Participants also completed established personality questionnaires. We found that participants demonstrated considerable flexibility, adjusting social information use to current conditions. In particular, individuals employed a ‘copy-when-uncertain’ social learning strategy, supporting a core, but untested, assumption of influential theoretical models of cultural transmission. Moreover, participants adjusted the amount invested in their decision based on the perceived reliability of personally gathered information combined with the available social information. However, despite this strategic flexibility, participants also exhibited consistent individual differences in their propensities to use and value social information. Moreover, individuals who favoured social information self-reported as more collectivist than others. We discuss the implications of our results for social information use and cultural transmission.

Cultural versus reproductive success: Why does economic development bring new tradeoffs?

Achievements that attract social rewards in developed countries, such as educational qualifications, a prestigious career, and the ability to acquire prestige goods, interfere with a womans ability to achieve reproductive success. This tradeoff between cultural and reproductive success may have developed because economic development creates an evolutionarily novel social environment. In the social environment of developed countries, a far smaller proportion of social exchange is between kin than in the small‐scale communities in which the human brain and behavior evolved. Evidence suggests that social interaction between non‐kin is less likely to encourage behavior that enhances inclusive fitness. A model of the cultural change that is likely to result from this change in social influence suggests that beliefs and values will become increasingly less consistent with the pursuit of fitness (Newson et al. [ 2007 ]: Evol Hum Behav 28: 199–210). Responses to the World Value Survey, which has been carried out in over 70 countries, confirm a number of the predictions of this model. In countries where fertility began to decline more recently, people appear to perceive the costs of having children to be lower relative to the cost of childlessness and the benefits of being a parent. Am. J. Hum. Biol., 2009.

Cultural group selection follows Darwin's classic syllogism for the operation of selection

The main objective of our target article was to sketch the empirical case for the importance of selection at the level of groups on cultural variation. Such variation is massive in humans, but modest or absent in other species. Group selection processes acting on this variation is a framework for developing explanations of the unusual level of cooperation between non-relatives found in our species. Our case for cultural group selection (CGS) followed Darwins classic syllogism regarding natural selection: If variation exists at the level of groups, if this variation is heritable, and if it plays a role in the success or failure of competing groups, then selection will operate at the level of groups. We outlined the relevant domains where such evidence can be sought and characterized the main conclusions of work in those domains. Most commentators agree that CGS plays some role in human evolution, although some were considerably more skeptical. Some contributed additional empirical cases. Some raised issues of the scope of CGS explanations versus competing ones.

Who Was Helping? The Scope for Female Cooperative Breeding in Early Homo

Derived aspects of our human life history, such as short interbirth intervals and altricial newborns, have been attributed to male provisioning of nutrient-rich meat within monogamous relationships. However, many primatologists and anthropologists have questioned the relative importance of pair-bonding and biparental care, pointing to evidence that cooperative breeding better characterizes human reproductive and child-care relationships. We present a mathematical model with empirically-informed parameter ranges showing that natural selection favors cooperation among mothers over a wide range of conditions. In contrast, our analysis provides a far more narrow range of support for selection favoring male coalition-based monogamy over more promiscuous independent males, suggesting that provisioning within monogamous relationships may fall short of explaining the evolution of Homo life history. Rather, broader cooperative networks within and between the sexes provide the primary basis for our unique life history.

Cultural Evolution and Human Reproductive Behavior

There is much that is “typically ape” about the way humans go about living their lives and building their babies (e.g., Chapais 2008; Smith and Tompkins 1995). Among mammals, apes and humans are characterized by low lifetime fertility, a slow pace of reproduction, and a large investment in each offspring. Many of the differences between apes and humans can be considered differences of degree and not kind. For example, like other apes, humans have a large brain in comparison to our body size. The human brain is extreme, however, being about three times the size of the chimpanzee brain. Since brain tissue is “expensive” to maintain and build (Aiello and Wheeler 1995; Isler and Van Schaik 2009a), humans have an even bigger problem providing our developing babies with sufficient nutrition. Much of what is not typically ape about humans is a consequence of the fact that human populations share a complex culture. In this chapter, I will describe the role of culture in human reproduction and suggest how the motivations and abilities that maintain culture could have coevolved with human life history and parenting behavior.

The limits imposed by culture: Are symmetry preferences evidence of a recent reproductive strategy or a common primate inheritance?

Womens preference for symmetrical men need not have evolved as part of a good gene sexual selection (GGSS) reproductive strategy employed during recent human evolutionary history. It may be a remnant of the reproductive strategy of a perhaps promiscuous species which existed prior to the divergence of the human line from that of the bonobo and chimp.