Trivers-Willard principle
The Trivers-Willard principle was described in 1973 by Robert Trivers and Dan Willard as a sociobiological effect resulting from the high variance in reproductive success in males. The principle states that mothers in better physical condition (e.g. nutritional status) should preferably invest in male offspring, while those with poorer physical condition should invest in female children. The principle proposed by the authors, if it existed, would contradict a famous prediction by the population geneticist Ronald Aylmer Fisher , according to which the investment in both sexes should normally always be the same.
A necessary prerequisite for the validity of the Trivers-Willard principle is that mothers can influence or determine the gender relationship of their offspring in some way. At first, no mechanism for this was foreseeable. Empirical studies have produced conflicting results. In a meta-analysis, the researcher Elissa Cameron found a certain tendency that mothers with a better nutritional status at the time of conception actually have male offspring a little more often than with an equal distribution statistically expected. As a hypothetical trigger, she suggests the glucose level.
Use in humans
Trivers and Willard established their principle generally for mammal species, but considered it to be at least partially applicable to humans. Later, the principle was also applied more generally to humans, whereby in addition to physical fitness characteristics such as nutritional status or height, social characteristics such as status must be taken into account. According to this, parents of higher status should invest more in raising boys, whereas parents of lower status should invest more in raising girls.
The reason for this is presumably that in the context of sexual selection , i.e. in the broadest sense of partner choice, men with good (material) resources have better chances with women, since they prefer fathers for their children who can ensure their survival. If a mother of low status invests in male offspring, she must assume that the son has little or no chance of finding a suitable partner when looking for a partner. This does not apply to the same extent to daughters. Girls' chances are maximized primarily through youthfulness and health, which can be promoted by good care, while resources such as status, money and influence play a lesser role in girls.
A high-status mother can assume that her male offspring will most likely find a suitable partner. Investing in male offspring is therefore worthwhile. The birth and rearing of a girl represents a certain risk for this mother, as the female offspring later have to compete with girls from lower status classes, which reduces the chances of reproductive success.
According to research by researcher Satoshi Kanazawa, abused women who are with violent men have significantly more sons than daughters. This observation is consistent with a generalized formulation of the Trivers-Willard principle. Ultimately, the principle depends on which men promise good reproductive success from the point of view of women. From an evolutionary point of view, these can also be aggressive men, because for the longest time in human evolution, aggressive men had a higher chance of successful reproduction. The sons of aggressive men, who also tend to be more aggressive themselves, “inherit” this advantage. According to the generalized Trivers-Willard principle, rearing sons is worthwhile for a woman with an aggressive partner.
literature
- Eckart Voland : The nature of man. CH Beck, Munich 2007.
Individual evidence
- ↑ Trivers, RL & Willard, DE (1973): Natural selection of parental ability to vary the sex ratio of offspring. Science, 179, 90-92.
- ↑ Elissa Z. Cameron (2004): Facultative adjustment of mammalian sex ratios in support of the Trivers – Willard hypothesis: evidence for a mechanism. Proceedings of the Royal Society London Series B 271: 1723-1728. doi : 10.1098 / rspb.2004.2773
- ^ S. Kanazawa: Violent men have more sons: further evidence for the generalized Trivers-Willard hypothesis (gTWH). In: Journal of Theoretical Biology. Volume 239, Number 4, April 2006, pp. 450-459, doi : 10.1016 / j.jtbi.2005.08.010 , PMID 16246373 .