メディエイゴ

Testosterone(1) and oestrogen(2) can cross membrane barriers(3) in the uterus(4). A female twin foetus(5) is therefore exposed to a brother's testosterone and a male twin foetus to a sister's oestrogen. However, male and female foetuses have similar oestrogen levels, so a female is more likely to be affected.

Certain characteristics, including facial features(6) and fertility(7), can be changed by exposure(8) to opposite sex foetuses. But until now little was known about how this effected future reproduction(9).

Dr Virpi Lummaa and colleagues from the University's Department of Animal and Plant Sciences used Finnish(10) church records spanning(11) 1734 to 1888 as part of their research. Of 754 twins from five distinct(12) Finnish populations in the records, females who survived to adulthood(13) were 25% less likely to have children if their twin was a male. Those who did have offspring(14) gave birth(15), on average(16), to two fewer babies than women who had a twin sister. Women who had a male twin were also 15% less likely to get married.

The researchers believe that this is due to(17) a number of(18) factors. Firstly, females exposed to a male twin can have masculine(19) traits(20), attitudes(21) and behaviours(22), therefore affecting their decision to get married or a male's attractiveness(23) towards them. Secondly, exposure to elevated(24) levels of testosterone during development(25) can promote(26) the onset(27) of diseases that compromise(28) fertility, such as reproductive cancers(29).