Genes that are late in life could explain why women have poorer health than older men, according to new research.
Scientists have long considered why older women are less healthy than older men, because men of that age are more likely to die than women (a puzzle known as "male and female paradox for survival in health").
The answer, according to University of Exeter scientists, is "intralocus sexual conflict genes that benefit one sex but harm others.
Researchers have used mathematical models and experimental data on flies to show that these genes can spread easily if they occur after stopping female reproduction.
"Common genes combine gender together in the evolutionary turn of the war," said Professor David Hosken of Exeter University.
"Selection tries to push women and men in different directions, but the shared genome means that each sex stops the other from reaching its optimum.
"Basically, certain genes will make a good man, but a bad woman and vice versa.
"However, after women reach menopause, they stop reproducing their genes, which means that women's selection (which is reproductive) is greatly weakened.
"So after that moment there will be some genes that will improve male fitness in the late life, even if they damage the female physical condition."
Professor Hosken said it is important to note that survival and health are not the same – and that the accumulation of male benefit genes in later life depends on the ability of men to continue reproduction after the age of female menopause.
Experimental data on flies (Drosophila) supported the detection of mathematical models in humans because genes that were better in men in late life tended to be much worse for women.
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C. Ruth Archer et al., Sexual Conflict Intralocus can solve the paradox of survival among men and women, Natural communication (2018). DOI: 10.1038 / s41467-018-07541-y