Trivers‐Willard Hypothesis and Human Parental Favouritism

This entry was written by Chloe Tan as part of a project done in BIAN 2133 ‘Human Reproductive Strategies’ at The Australian National University in 2019 Semester 2


It is a truth universally unacknowledged that parents do, to some extent at least, display parental favouritism. Studies across all taxa have shown that parents do discriminate amongst their offspring. This does not necessitate that parents consciously favour certain offspring over others. Evolved mechanisms for discriminative parental investment can work unconsciously. Parental investment is any investment by the parent in an individual offspring that increases the offspring’s fitness, while depleting the parent’s ability to invest in other offspring. I use the term ‘parental favouritism’ to refer to biases in parental investment between offspring. This essay focuses on sex-biased parental investment in humans, where sons may be favoured over daughters or vice versa.

Main text

Trivers-Willard Hypothesis

That a sex-biased pattern of parental investment evolved seems odd at first, because natural selection is expected to favour parents who invest equally into both sexes (Fisher, 1930). All else being equal, parents should be indifferent to offspring sex and not bias investment towards either sex. Using birth sex ratio (BSR) as a measure of parental investment in one sex over the other, the BSR is expected to be 1:1 when the costs and benefits to rearing sons and daughters are equal. In actuality, the costs and benefits to raising sons as compared to daughters may differ, resulting in a sex difference in rate of returns to investment.
The benefits of raising sons versus daughters may be viewed in terms of the offspring’s reproductive success. Typically, males have a greater variance in reproductive success than females. The most successful male in a population can dominate reproduction, siring majority of the offspring, while the least successful males sire none. Due to physiological constraints, females are usually less variable in their reproductive success. Generally, poor-condition females have a higher reproductive success than poor-condition males, and good-condition females have a lower reproductive success than good-condition males.

Trivers and Willard (1973) argued that natural selection can favour unequal parental investment between the sexes when there is a consistent sex difference in variance in reproductive success. The Trivers-Willard Hypothesis (TWH) predicts that mothers in poor maternal condition should produce more daughters than sons while mothers in good maternal condition should produce more sons than daughters in order to maximise fitness. Good maternal quality translates into high-quality sons. Preferentially investing in sons capitalizes on their high reproductive value. Poor maternal quality mothers benefit more from producing low-quality daughters that still have greater reproductive value than low-quality sons. The opposite may occur – good-condition mothers produce more females, while poor-condition mothers produce more males – when females have a higher variance in reproductive success than males.

Trivers and Willard (1973) originally used BSR as an indicator of sex-biased parental investment. They noted that in species where offspring require a long period of postnatal parental investment, parents might evolve behavioural adaptations facilitating sex-biased parental investment according to the TWH. Subsequent papers expand the TWH to account for postnatal sex-biased parental investment such as in educational investment, interbirth intervals, and nursing period (Cronk, 2007).

Problems surrounding TWH

Animal studies measuring BSR biases generally conform to the TWH. However, evidence for the TWH in human populations is mixed. Cronk (2007) argues that this is due to inappropriate applications of the TWH. He asserts that natural selection should result in the evolution of unconscious TWH-adhering cognitive modules or mechanisms, such as mechanisms for BSR adjustment. He noted that most human studies challenging the TWH use ill-fitting measures for both parental investment and maternal condition. Human studies have generally shown empirical support for sex-biased parental investment in humans when measured using BSR. One such study found that high-status parents (an indicator of maternal condition) were significantly more likely to produce more sons than daughters (Luo et al., 2016). Evolutionarily novel forms of parental investment such as inheritance of wealth might be governed more by economical, conscious calculations rather than by evolved mechanisms for parental investment.

Nevertheless, postnatal parental behaviour should not be definitively excluded from investigations. When critically studied, postnatal parental investment may be appropriate measures of parental investment. Nursing duration is a less controversial postnatal behavioural measure of maternal investment in children. Fujita et al. (2012) found evidence supporting the TWH in a study of breastfeeding mothers in Kenya. Economically more successful mothers were observed to nurse sons more frequently than economically less successful mothers, who instead spent more time nursing daughters. The biological composition of breast milk was different for mothers in similar conditions, when produced for sons versus daughters. Good-condition mothers produced milk with higher fat concentration for sons as compared to daughters while poor-condition mothers produced higher fat concentration milk for daughters as compared to sons.

Other models of parental favouritism

Local factors such as local mate competition (LMC) and local resource enhancement (LRE) can also alter the sex-specific costs and benefits of rearing offspring. Fig wasps are a classic example of LMC. A foundress mother lays her eggs within one fig and dies. Her offspring hatch and mate with one another. This is an example of LMC because sons experience competition amongst themselves to mate with their sisters. Mothers do not care which son fertilizes her daughters, because she is equally related to all her sons. The optimal strategy is to invest more in daughters and produce only just enough sons to fertilize all the females. As such, mothers in this species have been found to invest more in daughters, producing a female-biased sex ratio (Raja et al., 2008).

In humans, marriage payments, contribution to household work, and same-sex sibling competition for resources can instigate a sex-bias in parental investment. In an example of LRE in humans, Hutterite mothers were observed to invest in daughters more, nursing them for longer periods and having a longer interbirth interval after birthing females. Hutterite daughters were found to contribute to household work, thus making them cheaper to raise (Cronk, 2007). As daughters enhanced local resources (the abilities of their family members to survive and/or reproduce), investment in females, as opposed to males, garnered better return rates.


Evolved parenting adaptations concerns maximising the fitness of parents, which is not necessarily equal to offspring fitness. Parental favouritism, where some offspring benefit at the expense of others, may thus occur. This essay discusses how natural selection can lead to the evolution of sex-biases in parental investment under specified conditions. In studying delicate topics such as this, the naturalistic fallacy should be kept in mind – what is natural is not necessarily moral. Although certain parenting behaviour may be evolutionarily ‘fitter’, an appeal to nature should not serve as a guide to parenting.

Literature Cited

Cronk, L. (2007). Boy or girl: Gender preferences from a Darwinian point of view. Reproductive BioMedicine Online, 15(2), 23-32.

Fisher, R.A. (1930). The Genetical Theory of Natural Selection. Oxford, England: Clarendon Press.

Fujita, M., Roth, E., Lo, Y.-J., Hurst, C., Vollner, J., & Kendell, A. (2012). In poor families, mothers’ milk is richer for daughters than sons: A test of Trivers-Willard hypothesis in agropastoral settlements in Northern Kenya. American Journal of Physical Anthropology, 149(1), 52-59.

Luo, L., Zhao, W., & Weng, T. (2016). Sex-biased parental investment among contemporary Chinese peasants: Testing the Trivers-Willard hypothesis. Frontiers in Psychology, 7, 01215.

Raja, S., Suleman, N., Compton, S.G., & Moore, J.C. (2008). The mechanism of sex ratio adjustment in a pollinating fig wasp. Proceedings of the Royal Society B: Biological Sciences, 275 (1643), 1603-1610.

Trivers, R.L., & Willard, D.E. (1973). Natural selection of parental ability to vary the sex ratio of offspring. Science, 179(4068), 90-92.

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