Having a male twin makes the sister less likely to marry later in life.BANANASTOCK

Any woman with a brother could tell you that boys can be noisy, unruly and annoying. Now it seems that women with twin brothers have an extra reason to complain: they lower her chance of getting married.

Researchers have found that women who have a twin brother are also less likely to have children than women with twin sisters.

The study¹, conducted by Virpi Lummaa, an evolutionary biologist at the University of Sheffield, UK, shows that women who shared their mother's womb with a twin brother were about 25% less likely to have children when they reached adulthood. They were also about 15% less likely to marry and, if they did have children, they had fewer of them.

"Perhaps they were not as willing to marry," says Lummaa, "or perhaps they weren't as attractive to the opposite sex."

Hormone boost

The study was done by examining birth and marriage records from five Finnish church parishes between 1734 and 1888. Lummaa used these old records, as opposed to contemporary data, because they provide a good example of birth rates before the advent of modern interventions such as contraception and in vitro fertilization, Lummaa notes. "The norm back then was to marry," she says. "And, for the most part, everyone who could have children reproduced once they were married."

Lummaa suggests that the lower fertility rates in women who had a male cotwin was the result of being exposed to higher levels of the male sex hormone testosterone while still in the womb.

Male fetuses produce higher levels of testosterone than female fetuses. Testosterone is responsible, among other things, for the development of the male sex organs in utero. And, because testosterone can be transferred from fetus to fetus through the amniotic fluid, girls who share a womb with their twin brother are exposed to higher levels of the hormone during development.

Animal magnetism

Lummaa had no way of actually testing the hormone levels of the people she studied, but the effects of prenatal exposure to testosterone is well studied in animals.

Fred vom Saal, a developmental endocrinologist at the University of Missouri-Columbia, points out that female mouse fetuses that develop between two male fetuses (and are thus exposed to higher levels of testosterone), are less adept at mating later in life.

"If you give a male mouse a choice between a female who hasn't been exposed to testosterone in the womb and a one who has, he will always choose the female that was exposed to the lower levels of testosterone," he says.

In addition, says vom Saal, if a male mouse does attempt to mate with them, female mice who have been exposed to prenatal testosterone are simply not as receptive to sexual advances. "They'd rather fight than have sex," he says.

So vom Saal isn't surprised that similar results have been found in humans. "If you look at the way the hormonal systems work, they are very, very similar to each other," he says.

Lost twins

Lummaa argues that the lower reproductive success among women who have a twin brother isn't due to other effects such as poor nutrition or preferential treatment of male infants. In the study, she also looked at women whose twin brother had died shortly after birth and who had been raised as a single child. She found that they had the same chance of marrying and reproducing as did women whose twin brother had survived.

Vom Saal points out that the lower levels of marriage and reproductive success does not necessarily mean that these women are unfit in an evolutionary sense. In mice, females who have been exposed to higher levels of prenatal testosterone may produce fewer offspring but, he adds, they tend to be more vigilant and protective mothers.

"The evolutionary argument is that you may have fewer kids, but there's a greater chance that they'll survive," he says.

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• References

  1. Lummaa V., Pettay J. E. & Russell A. F. Proc. Natl Acad. Sci. USA, doi:10.1073/pnas.0605875104 (2007).