When a son and girl are conceived together , the distaff twin is disclose to more manlike hormones in the uterus than when both twins are girls . This has allowed researchers to essay the much - fence impact of these internal secretion prior to birth . They have found that there is a small , but statistically substantial , minus impact on life history outcome from nine months in close confinement with a male child .
Animal survey have shown females that share a litter with males dissent from those from all distaff - litters . If such differences exist in humans , however , they must be subtle ; you ca n’t look at an individual woman and tell if she has a manly counterpart ( a rare case of scientific accuracy inStar Wars ) .
The implausibly elaborated disk - keeping of Norse countries allowedDr Krzysztof Karbownikof Northwestern University to essay for statistical issue . Together with Norse scientist , Karbownik read the life-time outcomes of 13,717 twins accept in Norway between 1967 and 1978 .
InProceedings of the National Academy of Sciences , Karbownik reports girls with a duplicate brother are less potential to fine-tune from gamy school or university and have fewer children and low earnings into their 30s than those with duplicate sisters . The differences are limited – average out around 10 percentage – but large enough to support a real issue .
The next interrogation for Karbownik was whether the difference arose from growing up with a brother of the same age , or if it reflects pre - natal hormones , a theory do it as thetwin testosterone - transferral surmise .
The squad looked for a sample of girl whose similitude had died in babyhood . Even with Norway ’s excellent health organisation , this was disturbingly coarse at the time , giving a sample distribution large enough to show it is have a chum in the womb , rather than in life , that makes the difference . On the other hand , relative weight at birth had no influence on effect , contradict theories about manly twins catch scarce nutrient .
The study did n’t look at male child with twin sister , but there is little evidence non - human male animate being are dissemble .
" This is the first subject field to track citizenry for more than 30 old age , from birth through schooling and adulthood , to show that being expose in utero to a manlike duplicate influences important outcomes in their twin sister , including school graduation , pay , and fertility rate , ” Karbownik said in astatement .
It ’s not the first field of study to detect a potential negative force for twin fille that partake a womb with a brother . A2007 studycarried out by Dr Virpi Lumma from the University of Sheffield , using the health record of Finland , observe that girls with duplicate brothers were 25 percent less likely to have nipper if their twin was male . The team suspected exposure in the womb to testosterone may damage fertility .
Twin numbers , include interracial - sex activity pairs , are rising because of IVF , but the authors do n’t think the consequences are declamatory enough to raise alarum . They add there may be even out electropositive effects they have not measured , and changing gender norms may make what was a disadvantage in the ' seventy into something more positive today .
Still , we would n’t want to be the 1 to tell Cersei Lannister though .
