Sexual conflict and the Trivers-Willard hypothesis: Females prefer daughters and males prefer sons.

Because parental care is expected to depend on the fitness returns generated by each unit of investment, it should be sensitive to both offspring condition and parental ability to invest. The Trivers-Willard Hypothesis (TWH) predicts that parents who are in good condition will bias investment towards sons, while parents who are in poor condition will bias investment towards daughters because high-quality sons are expected to out-reproduce high quality daughters, while low-quality daughters are expected to out-reproduce low quality sons. We report results from an online experiment testing the Trivers-Willard effect by measuring implicit and explicit psychological preferences and behaviorally implied preferences for sons or daughters both as a function of their social and economic status and in the aftermath of a priming task designed to make participants feel wealthy or poor. We find only limited support for predictions derived from the TWH and instead find that women have strong preferences for girls and men have preferences for boys.

Resource conflict and cooperation between human host and gut microbiota: implications for nutrition and health.

Diet has been known to play an important role in human health since at least the time period of the ancient Greek physician Hippocrates. In the last decade, research has revealed that microorganisms inhabiting the digestive tract, known as the gut microbiota, are critical factors in human health. This paper draws on concepts of cooperation and conflict from ecology and evolutionary biology to make predictions about host-microbiota interactions involving nutrients. To optimally extract energy from some resources (e.g., fiber), hosts require cooperation from microbes. Other nutrients can be utilized by both hosts and microbes (e.g., simple sugars, iron) in their ingested form, which may lead to greater conflict over these resources. This framework predicts that some negative health effects of foods are driven by the direct effects of these foods on human physiology and by indirect effects resulting from microbiome-host competition and conflict (e.g., increased invasiveness and inflammation). Similarly, beneficial effects of some foods on host health may be enhanced by resource sharing and other cooperative behaviors between host and microbes that may downregulate inflammation and virulence. Given that some foods cultivate cooperation between hosts and microbes while others agitate conflict, host-microbe interactions may be novel targets for interventions aimed at improving nutrition and human health.

Imitation is necessary for cumulative cultural evolution in an unfamiliar, opaque task.

Imitation, the replication of observed behaviors, has been proposed as the crucial social learning mechanism for the generation of humanlike cultural complexity. To date, the single published experimental microsociety study that tested this hypothesis found no advantage for imitation. In contrast, the current paper reports data in support of the imitation hypothesis. Participants in "microsociety" groups built weight-bearing devices from reed and clay. Each group was assigned to one of four conditions: three social learning conditions and one asocial learning control condition. Groups able to observe other participants building their devices, in contrast to groups that saw only completed devices, show evidence of successive improvement. These results are consistent with the hypothesis that imitation is required for cumulative cultural evolution. This study adds crucial data for understanding why imitation is needed for cultural accumulation, a central defining feature of our species.