Girls With Twin Brothers Apparently Pay a Price For Sharing a Womb With a Boy

Image of male female fraternal twins.mgeejnr via Pixabay

Twins often encounter cheerful conversations about what it is like to grow up with one and how much they are alike or different. And of course, there’s always the twin bond that everyone talks about.

However, being one half of a fraternal, two-gender pair may not be harmonious as it seems – in the womb. A recent study published in PNAS examined the prenatal impact of male twins on female twins. What does that mean?

It is being asserted that females who shared the womb with a male twin have less advantages when it comes to future income, marriage rates, education, and fertility. Researchers examined data for over 13,000 twins born in Norway between 1967 and 1978 and found some quality of life differences for male-female fraternal sets.

Specifically, “. . .females exposed in utero to a male co-twin have a decreased probability of graduating from high school (15.2%), completing college (3.9%), and being married (11.7%), and have lower fertility (5.8%) and life-cycle earnings (8.6%).”

Scientists from Northwestern University and the Norwegian School of Economics analyzed data for these twins born during a 12-year period. One of the authors, Krzysztof Karbownik, stated:

“This is the first study to track people for more than 30 years, from birth through schooling and adulthood, to show that being exposed in utero to a male twin influences important outcomes in their twin sister, including school graduation, wages and fertility rates.”

Compared to women who had twin sisters, those with twin brothers didn’t fare as well in life. This held true even in circumstances where the male twin died in utero or shortly after birth. Scientists are attributing this to two factors: exposure to testosterone in the womb and possibly, competition for nutrients.

Researchers want to point out that this scenario is not necessarily true for women with a male twin. Other biological, behavioral, and societal factors play a role in these outcomes. Therefore, it is not a given that a female with a male twin will earn less, have no children (or a small number of them), or have difficulty finishing school.

Although more research is needed, it is possible that being exposed to testosterone in the amniotic fluid or umbilical cord may have some benefits. One of the authors also cautioned that cultural norms and the prevailing social climate can impact the outcome of these women’s lives:

“It is important to emphasize that our findings apply to Norwegian society during the timeframe of the study, but may not apply equally across other societies or cultural settings. For instance, if gender norms change within a society, acceptance of a wider array of behaviors could minimize later effects on outcomes like school completion or entering a marriage.”

Women out there who have a twin-ship like the one at the base of this study might be examining your own lives after reading this, but be gentle. As these researchers stated, your twin brother doesn’t hold all the cards when it comes to how life turns out.

Are you part of a male/female twin pair? What do you think of these findings? Do you know of any twins whose lives are the opposite of this study?


Science Daily