Twins have long been hailed as one of nature’s greatest boons to genetic research. Born into the world in tandem, they represent one of evolution’s most elegant experiments: Two individuals who share parents, a uterus, a birthdate, and, often, the circumstances of childhood.
But often more striking than what twins have in common are the differences between them—or, perhaps, how they might change each other. According to a study published today in the journal PNAS, the biological sex of a baby girl’s twin might alter her life trajectory: Compared to women who shared the womb with a sister, females with twin brothers earn lower wages, have fewer children, and are less likely to get married, be employed, or graduate from high school or college.
Without following up with every single individual, it’s hard to know what underlies these outcomes, says study author Christopher Kuzawa, an anthropologist at Northwestern University, but “our best guess is that what’s changing is primarily behavior.”
Though the root causes of these differences remain unclear, the discrepancies held true even for females whose twins died shortly after birth. Presumably, this rules out the effects of growing up alongside a sibling of the same age—and hints at the possibility that these women might be experiencing long-term biological results of sharing the womb with a brother.
“This is a well-powered study, and the effects are striking,” says Margaret McCarthy, a neuroscientist who studies sex and gender differences in the brain at the University of Maryland and was not involved in the study. “But it’s important that these consequences aren’t painted as deleterious…instead, this should be framed in terms of natural variation.”
To analyze birth, household composition, schooling, and employment data, the researchers examined records of 13,700 twins born between 1967 and 1978 in Norway, which, like all Nordic countries, keeps detailed nationwide registries of its population. “This is the largest study of its kind,” Kuzawa says. “We were able to look at everyone born in Norway over a 12-year period.”
Based on the metrics recorded by the Norwegian government, the researchers determined that women with a biologically male, rather than female, twin were 15.2 percent less likely to complete high school, and 3.9 percent less likely to graduate from college. Additionally, at age 32, these women were 11.7 percent less likely to be married and had, on average, 0.09 fewer children. By that age, they were also 3.2 percent less likely to be in the labor force, and those that were had 8.6 percent lower salaries. There was no effect on a final factor—whether or not women chose a STEM or economics major in college.
This isn’t the first study to look into how male siblings might shape their twin sisters’ life outcomes, Kuzawa says, but most previous work hasn’t been able to disentangle two theories about how these changes come about: through the social effects of being raised with a brother, or a biological spark that might occur in utero.
With thousands of records to sift through, however, the researchers were able to repeat their analyses on a subset of 583 women whose male twins died within the first year of life (and again for the 239 of those whose twins died within the first month) and had thus been raised without their brothers.
Both smaller-scale experiments yielded similar—but not identical—results. “The [number of people analyzed] really impacts the significance of the findings,” says Jaclyn Schwarz, a neuroscientist who studies the effects of early development on behavior at the University of Delaware and was not involved in the study. The analyses’ statistical power, or what “allows you to say you’re not making a conclusion that’s not actually there,” went down when the researchers looked at a more limited study population, she says.
Still, even with these much smaller datasets, the overall trends remained, Kuzawa argues, suggesting that the drivers behind these phenomena were taking hold before birth.
This, it seems, would implicate something in the uterus. But that’s where things get murky. Most of the experiments that could pinpoint the source of these differences can’t be done ethically in humans, explains study author Krzysztof Karbownik.
Currently, the researchers’ leading hypothesis is one that’s found some traction in rodent models: testosterone is spilling over from male twins to their sisters in the womb, leading to behavioral changes down the road. This hasn’t been proven, Karbownik says, but “that’s what biology says should happen.”
Others are more skeptical. “It seems unlikely to me that it’s prenatal testosterone exposure,” says Melissa Hines, a developmental psychologist and expert in gender-related behavior at the University of Cambridge in the United Kingdom who was not involved in the study. “The findings don’t fit with what you’d predict about the effects of testosterone on sexual differentiation, or how it influences human development.”
Many differences exist between the male and female fetal environment, raising the possibility of other explanations, says McCarthy, the University of Maryland neuroscientist. For instance, mothers mount different immune responses to male and female fetuses; having both in the uterus at the same time could create an unusual set of circumstances for the female twin, irrespective of hormonal input.
Regardless of the biology at play, if behavior is what’s mediating these social and economic outcomes, that might raise its own slew of questions. The researchers’ results don’t imply that these women are inherently ill-equipped for success, but they could indicate that they’re experiencing consequences from simply behaving differently.
This might be especially true if the women came of age in a society with highly restrictive and unforgiving gender norms, McCarthy says. Individuals who don’t behave within the bounds of certain constraints might suffer feelings of exclusion—or even be explicitly penalized for their nonconformity—and feel more compelled to leave school early, for instance.
Because of this cultural contingency, “the effects we’re documenting here cannot be extrapolated to other nations or periods of history,” Kuzawa says. “The impacts of behaviors depend on the society the person lives in…and cultural and gender norms are variable across societies, and even change within societies. Something not acceptable 30 years ago could be more acceptable today.”
These caveats are important to keep in mind given the near doubling in rates of twin births since 1980—spurred on by an increase in the average age of conception in women, as well a rise in the use of fertility drugs and assisted reproductive technology. For now, there’s no cause for concern, Kuzawa says, and the researchers’ results don’t advocate for or against any reproductive decisions over others.
In the meantime, it does no good to see these social and economic outcomes as one-dimensional “negatives,” McCarthy says. “In a society that is more diverse and perhaps more accepting or tolerant of more fluid gender norms, there could be hidden advantages for these girls.”
Even within the study population, there’s no evidence that these women are unhappy, or even that they’re not thriving. “We shouldn’t suggest that any slight variation in a woman’s behavior puts her at a disadvantage,” McCarthy says. After all, she adds, all of these results could simply be chalked up to life choices that women have made—like deciding to have fewer children—that have no bearing on their “success” in life.
“Your mind might read this and think it constitutes a ‘deviation from normal,’ but this isn’t abnormal behavior,” Schwarz, the University of Delaware neuroscientist, says. “Data like this falls on a spectrum, and what we’re looking at is a shift in the spectrum.”
Part of what’s going on here, Kuzawa says, might be a lesson in tolerance. After all, like many other aspects of identity, gender is not binary, but “plastic and fluid,” he says. “A lot of factors come together in different ways in different individuals. All of this is just a teeny lens into the vastness of human diversity.”