Imagine two babies developing side by side for months. One is a boy, the other a girl. They share the same pregnancy and the same maternal environment, yet their developing bodies produce different patterns of sex hormones.
Could one twin’s biology subtly affect the other before either of them is born?
It sounds like the premise of an unusual experiment. In reality, opposite-sex twins have provided scientists with a natural opportunity to investigate this question for decades.
The idea even has a name: the Twin Testosterone Transfer hypothesis. It proposes that a female fetus developing alongside a male twin might experience greater prenatal exposure to androgens, potentially leaving subtle biological or behavioral traces later in life.
Then, in 2019, a large Norwegian study produced results that attracted attention far beyond endocrinology.
Women who had twin brothers appeared to differ from women who had twin sisters not only in reproductive outcomes, but also in education, marriage and earnings.
The findings generated provocative headlines suggesting that girls somehow “pay a price” for sharing the womb with a brother.
But there is a major problem with turning that idea into a biological fact.
Scientists still have not established that the proposed testosterone transfer actually occurs in humans to the degree required to explain those outcomes.
And several studies have produced results that complicate the story considerably.
A remarkable Norwegian natural experiment
The 2019 research, published in the Proceedings of the National Academy of Sciences, used Norwegian population records covering more than 728,000 births between 1967 and 1978, including roughly 13,800 twins.
That scale matters.
Instead of recruiting a relatively small group of volunteers, the researchers were able to examine outcomes across a very large population.
When women from opposite-sex twin pairs were compared with women who had twin sisters, several differences emerged.
According to the study, women with twin brothers were 15.2% less likely to complete high school and 3.9% less likely to complete college. Their probability of being married was 11.7% lower, their fertility was 5.8% lower, and their lifetime earnings were 8.6% lower.
Those numbers sound dramatic.
But they require an important qualification: they describe average statistical differences between groups, not what will happen to an individual woman.
Having a twin brother does not mean a woman will earn less, have fewer children or be less likely to marry.
The far more interesting question was why these population-level differences appeared at all.
The obvious alternative explanation: growing up with a boy
Suppose researchers discover differences between women who have twin brothers and women who have twin sisters.
There is an immediate problem.
Those women didn’t merely share a uterus with a boy or girl.
Most of them also spent their childhood with that sibling.
A girl growing up alongside a boy may encounter different toys, friends, activities and expectations. Siblings influence one another, and parents and other adults may respond differently to a boy-girl twin pair than to two girls.
How could researchers distinguish nine months of prenatal biology from nearly two decades of shared childhood?
The Norwegian dataset offered an unusual opportunity.
The researchers separately examined women whose twin brothers had died during infancy.
These girls had experienced an opposite-sex twin pregnancy but had little or no opportunity to grow up alongside their brothers.
The dataset included 583 women whose twin brothers died within the first year of life and 239 whose brothers died during the first 28 days.
The associations did not disappear in these groups.
That finding weakened a purely social explanation and was one reason the researchers considered a prenatal mechanism plausible.
But “plausible” is an important word here.
It is not the same as “proven.”
Why scientists started looking at testosterone
The theory did not originate with Norwegian income statistics.
Researchers had already observed intriguing prenatal effects in animals.
In some species that carry multiple fetuses, an individual’s position relative to male and female fetuses can influence hormonal exposure and, in some cases, later characteristics.
That raised an obvious question about humans.
A male fetus produces androgens during development. If a female twin develops nearby, could some of those hormonal signals influence her too?
This became known as the Twin Testosterone Transfer hypothesis.
It is an elegant explanation.
Unfortunately, biology does not have to obey elegant explanations.
Human twins are not simply sharing one pool of blood
The phrase “sharing a womb” can create a misleading mental picture.
A boy-girl twin pair is normally dizygotic, meaning that two separate eggs were fertilized by two separate sperm cells.
Opposite-sex twins are generally dichorionic and diamniotic: each fetus has its own chorion and amniotic sac, and they generally develop with separate placental structures, although the placentas can sometimes appear fused.
They are therefore not simply two fetuses connected to one shared bloodstream.
Any meaningful transfer of hormones between them would have to occur through a more complicated biological pathway.
This anatomical reality is one reason findings from animals cannot automatically be transferred to human pregnancy.
And it leads to an important test.
If girls with twin brothers really experience greater androgen exposure, can researchers actually measure evidence of it?
Researchers measured the hormones — and found something unexpected
A prospective study published in 2020 examined cord blood from dichorionic-diamniotic twin pregnancies.
Researchers compared 20 girls who had twin brothers with 20 girls who had twin sisters and measured total testosterone and other androgens.
The expected testosterone difference wasn’t there.
Girls with twin brothers did not have higher cord-blood testosterone concentrations.
An earlier prospective endocrinological study had also failed to find higher androgen concentrations in the cord blood of females from opposite-sex twin pairs compared with females from same-sex pairs.
These findings do not completely disprove prenatal hormonal transfer.
Cord blood is collected at birth. A measurement taken then cannot perfectly reconstruct hormone exposure during every earlier stage of fetal development. It remains theoretically possible that differences occurred during a critical developmental window and were no longer detectable at delivery.
Still, this distinction is crucial.
There is a considerable difference between saying:
“Researchers have proposed that testosterone might be involved”
and saying:
“A twin brother exposes his sister to extra testosterone in the womb.”
The first reflects the evidence.
The second currently goes beyond it.
Some studies nevertheless find intriguing differences
The mystery persists because not every study produces a null result.
Researchers have reported differences between women with twin brothers and women with twin sisters in several areas.
One study involving 200 women with twin brothers found better performance on a mental rotation task — the ability to visualize objects being rotated in space — compared with women who had twin sisters. The authors considered the result consistent with possible prenatal androgen effects.
Other research has examined sensation-seeking behavior and reported differences in some measures.
Scientists have also studied the ratio between the lengths of the index and ring fingers, known as the 2D:4D ratio. It has sometimes been used as an indirect marker of prenatal hormonal exposure, although its reliability for reconstructing an individual’s prenatal androgen environment remains debated.
Some earlier twin studies produced results that appeared compatible with the testosterone-transfer hypothesis.
Others did not.
And that inconsistency is important.
When scientists looked across dozens of studies, the picture became messy
A scientific hypothesis becomes much more convincing when different researchers, populations and methods repeatedly produce compatible findings.
That has not happened cleanly here.
A broad review published in Neuroscience & Biobehavioral Reviews examined 60 studies comparing same-sex and opposite-sex twins.
Evidence considered compatible with the Twin Testosterone Transfer hypothesis appeared most frequently in research involving cognition. Even there, however, findings were inconsistent.
Evidence involving physiological and behavioral characteristics was weaker and more contradictory.
An earlier review had reached a broadly similar position: some findings involving perception, cognition and morphology were intriguing, but behavioral evidence was limited.
In other words, decades of research have produced clues rather than a biological verdict.
What about the claim that women with twin brothers have fewer children?
This is particularly important because reduced fertility was one of the striking findings in the Norwegian research.
A later study examined this question using maternal and neonatal records from Aberdeen, Scotland.
Researchers identified 317 dizygotic female twins: 151 had twin brothers and 166 had twin sisters. Their reproductive histories could be followed for approximately four decades.
The result did not reproduce the apparent reproductive disadvantage.
Women with twin brothers had similar chances of becoming pregnant and having children to women with twin sisters.
The researchers concluded that their results did not support an adverse effect of the proposed testosterone transfer on female reproductive outcomes.
That Scottish study was far smaller than the Norwegian population analysis, so it cannot simply erase the earlier findings.
But the disagreement demonstrates exactly why replication matters.
One population can reveal a fascinating association. Another can show that the association is not universal.
More recent research still hasn’t settled the question
Scientists continue to test predictions generated by the hypothesis.
A small exploratory study published in 2025 examined testosterone and the 2D:4D finger-length ratio in young adult twins.
Only 40 people participated, so the study was far too small to settle a decades-old debate.
Nevertheless, researchers did not find a significant difference in 2D:4D between women with twin brothers and women with twin sisters. Findings involving women’s testosterone levels also failed to provide strong support for the predicted effect after appropriate statistical corrections.
The authors raised several possibilities, including one that science always has to consider: perhaps the effect is simply too small to detect reliably — or perhaps the proposed effect does not exist in the way researchers originally imagined.
The Norwegian findings can be real without testosterone being the explanation
This is one of the most important lessons in the entire story.
A statistical association and its proposed biological explanation are two different discoveries.
The Norwegian researchers found population-level differences. That finding does not automatically establish the mechanism responsible for them.
There is another complication worth remembering.
The women in that study were born between 1967 and 1978.
Educational opportunities, gender expectations, employment patterns, marriage and decisions about having children have changed substantially since then.
Outcomes such as lifetime earnings, university education and marriage are also shaped by countless social, economic and personal factors.
None of this proves that prenatal biology played no role.
It simply means we should resist converting an intriguing association into a deterministic biological story.
What these studies do not tell us about individual women
A woman with a twin brother should not read this research as a prediction about her life.
Current evidence does not establish that having a male twin makes a woman less feminine, less fertile, less likely to marry, less academically successful or destined to earn less money.
Nor does it establish a particular personality type.
Population averages cannot be used as personal forecasts.
Imagine that researchers discover that people in one population group are, on average, slightly taller than those in another. Knowing which group two individuals belong to would still not reliably tell you which of those two people is taller.
The distributions overlap.
The same principle applies here, particularly when the measured outcomes are influenced by biology, family, culture, education and individual choices simultaneously.
Twins offer scientists an extraordinary natural laboratory
Despite all these uncertainties, opposite-sex twins remain scientifically fascinating.
Two fetuses develop during the same pregnancy, inside the same mother and during exactly the same historical period. After birth, they frequently share a household, socioeconomic environment and much of their childhood.
Yet one is male and the other female.
That combination gives researchers an unusual opportunity to ask questions that would be impossible — and unethical — to test experimentally in humans.
But it also creates the central difficulty.
Prenatal biology and postnatal environment are extraordinarily difficult to disentangle.
The Norwegian study’s examination of twins whose brothers died shortly after birth was an ingenious attempt to do exactly that.
It strengthened the argument that something prenatal might be involved.
It did not identify that “something” conclusively as testosterone.
The most interesting answer is still an unfinished one
There is good evidence that prenatal hormones matter for human development. There is also compelling animal research showing that neighboring fetuses can influence one another under certain reproductive conditions.
What remains uncertain is whether a human male twin exposes his sister to enough additional testosterone to produce measurable effects later in life — and whether such an effect could explain the social and reproductive patterns reported in some twin studies.
Some findings fit the hypothesis.
Others do not.
Direct hormone measurements at birth have not revealed the clear androgen difference the simplest version of the theory might predict. Fertility studies have produced conflicting results. Reviews covering dozens of studies describe an inconsistent body of evidence.
That makes the real story more interesting than the viral headline.
A girl and her twin brother can spend the first months of their existence only centimeters apart. It is biologically plausible that aspects of their prenatal environments could interact in subtle ways.
But after decades of investigation, scientists still cannot say with confidence how much one twin’s hormonal environment affects the other, whether the proposed testosterone effect is meaningful in humans, or whether it explains the differences observed in some populations.
Science rarely progresses from one spectacular study to a permanent answer.
More often, one study discovers an unexpected pattern. Another proposes a mechanism. Someone else measures the mechanism directly. A different population fails to reproduce the original result. New experiments refine the question again.
And occasionally, the most scientifically accurate conclusion remains the most intriguing one:
Something interesting may be happening before birth. We are still working out exactly what it is.