Are parents naturally biased towards their sons or daughters, depending on their conditions?

– by Valentin Thouzeau

According to Robert Trivers and Dan Willard’s hypothesis, in many species, parents in good condition should favour male offspring, while parents in poor condition should favour female offspring. Why? First, parents with more resources can support more offspring. Second, in polygynous species, males with more resources are more likely to have many offspring. Natural selection should therefore favour investment in male offspring when parents are in good condition, since their sons will have a chance to have many children in turn. Conversely, natural selection should favour investment in female offspring when parents are in poorer condition, since their daughters have a high chance of having children even if they do not have many resources (see Figure 1 for a schematic representation of this hypothesis). This prediction was tested in a numerous non-human species. For instance, in ungulates, the results showed that when a female’s partner can invest more resources in her offspring, she does indeed produce more sons.

Figure 1 – a) Example of a group in which males with many resources (represented by large circles) are likely to have more offspring than females with many resources, since they can have children with multiple females. b) Representation of the optimal choice in terms of investment by parents in a population represented in a. Parents with few resources should favour their female offspring, while parents with many resources should favour males. c) Prediction of the Trivers-Willard hypothesis arising from the choice in b. The sex ratio of offspring from parents with few resources should favour females, while it should favour males for parents with many resources.

But can this prediction be applied to humans? We often think we are above biology – yet some results seem to contradict this: one study showed that American millionaires have an average of 60% sons and 40% daughters! Does this prediction apply to how we treat our children? In another U.S. study, no difference was found between parents based on socioeconomic status in the amount of time they spend with their sons or daughters. So the results are uncertain.

At present, there are literally hundreds of studies testing the hypothesis in humans, and it is difficult to get a comprehensive view. Also, can we trust the proportion of results that support the Trivers-Willard hypothesis? It may be that researchers were more inclined to publish results that supported the hypothesis rather than those that invalidated it, which may have led to publication bias. We were surprised to find that results from studies involving many groups of animals were synthesised (this is the case, for example, for ungulates and non-human primates), but no synthesis of the work done in the human species had been undertaken until now. This is why we started this work.

We collected 87 studies reporting a total of 821 tests of the Trivers-Willard hypothesis. Although the majority of the samples were based in North America, the geographic coverage was considerable. The analysis of all these studies reveals that the results are largely compatible with the Trivers-Willard hypothesis and that there is no publication bias in this scientific literature that could alone explain the compatibility of the results with this hypothesis.

We then tested whether the hypothesis held true in the birth bias of boys and girls, in the investment bias that boys and girls receive after birth, or in both. We found that tests for both versions are equally prevalent in the scientific literature. However, the results indicate that birth bias is better supported than investment bias. Putting together the last 50 years of research on the Trivers-Willard hypothesis thus allows us to conclude that parents in good condition have, on average, more sons, while parents in poor condition have more daughters.

We have to keep in mind that even if the Trivers-Willard hypothesis is validated for the birth bias, this bias is very small (the correlation coefficient is 0.037). There is simply a very small additional probability of giving birth to sons when conditions are favourable, and to daughters when conditions are less favourable. Nevertheless, these results show how the theory of evolution leads to surprising predictions that allow us to discover unsuspected phenomen.

Read the original paper: Thouzeau, V., Bollée, J., Cristia, A., & Chevallier, C. (2023). Decades of Trivers-Willard research on humans: What conclusions can be drawn? Evolution and Human Behavior, 44(4), 324-331.