Environmental Influence

What genetic influences occur in utero with fraternal twins of opposite sexes? Does the male's testosterone have an effect on the developing female and vice versa? I seem to be ambidextrous with some tasks and my index finger is shorter than my ring finger.

-A curious adult from California

May 15, 2008

Scientists have long wondered about the effects that twins have on each other in the womb. Particularly boy-girl twins.

Unfortunately, there isn't a clear answer yet. Lots of studies have been done, but they give different answers. Some studies say that twins affect each other but others say they don't.

Scientists have trouble answering these kinds of questions because it is hard to separate the effects of developing in the womb from growing up as a twin. What this means is that figuring this out is still a work in progress.

Before getting too deep into all of this, let's address your specific questions about handedness and finger length ratios first. They are both traits that are determined during development in the womb.

So did having a twin brother lead to your ambidexterity and shorter index finger? From what we know so far, the answer is probably no (see below). Having a twin brother likely had no effect on your hand and finger development.

But we're still not 100% sure because scientists are still working on these questions. They're also trying to figure out if being a twin influences other traits too.

Before we talk about those, though, let's take a step back and ask how your twin brother might have given you specific traits. And from that try to understand why it is so hard to figure out IF he gave any traits to you.

The most likely way for a twin to affect another twin during development is through hormones. In your case, the hormone we're talking about is testosterone.

Having different amounts of testosterone in the womb can affect development. This includes traits like finger-length ratios, sporting ability, aggressiveness, and handedness.

So why should we try to pin these traits on your twin brother? Because during certain stages in his development, he was a testosterone making machine.

Each of us becomes male or female because of the hormones testosterone and estrogen. They give signals to our bodies to develop male and female parts.

For the first six weeks, boys and girls develop exactly the same. At the end of the sixth week, boys make a burst of testosterone. This testosterone turns on genes that end up making the fetus into a boy.

But boys aren't the only ones making testosterone. Girls make it too. They just make less of it.

Girls make testosterone because it doesn't just turn on genes that make someone a boy. Testosterone can also turn on other genes. Like ones involved in finger development.

Males usually have shorter index fingers compared to their ring finger. Women usually have longer index fingers compared to their ring fingers.

So scientists think that the ratio of your index finger to your ring finger indicates how much testosterone you were exposed to during development. But not everyone makes the same amount of testosterone.

This means some women will make more than others and have shorter index fingers. Even when they aren't sharing a womb with a boy. This can make it hard to figure out if a trait came from having a boy twin or just because you made more testosterone than average.

In fact the results from a recent study suggest that having a boy twin does not affect a girl's finger ratios. Researchers looked at 867 people and saw no difference in finger lengths whether a girl had a boy twin or not. So the boy twin can't affect the girl, right? Not so fast...

For example, scientists see a big effect in mice of having a boy twin. Why did they study mice?

Well, mice have about 6-8 babies each time they get pregnant. Plus, mice develop in a single row, just like peas in a pod. So scientists can easily tell if a pup was surrounded with females or males in the womb.

The researchers found that female pups surrounded with male pups had more male traits, like aggressiveness. That means the testosterone from the male pups can affect the development of the female pups.

And some studies show that human women can be affected by a male twin too. For example, one study found that the women from boy-girl twins were less likely to get married and 25% less likely to have kids. At least that was true in Iceland from 1734-1888. The authors suggest that this might have been caused by extra testosterone in the womb.

The girl can also affect the boy twin. A study found that boys with twin sisters were more likely to become anorexic. Since anorexia is much higher in females, perhaps the extra female hormones affected the male twins during development.

So twins do affect each other right? Well certainly being a twin does. But as I said at the beginning, the tricky part is separating out the effects before a twin is born from the effects after he or she is born.

Some traits, like finger length ratio, are determined by our genes and exposure to hormones during development. But other traits like having kids, getting married, and being anorexic can be affected by being raised with a twin too. Just think of how different your life would be if you didn't have a twin brother to spend all your time with.

For example, boys are often more aggressive than girls. So you may learn from and copy his aggressiveness. Or, you may have to be tougher just to deal with him! Either way, being raised with a twin brother may make you more aggressive.

So we still don't have a good answer as to whether sharing a womb with a twin of the opposite sex affects someone or not. There do seem to be some effects. We just can't yet separate out which are due to developing together in the womb. And which are due to being raised with a twin.

One way to answer this would be to find boy-girl twins who were raised separately. Or families that had adopted a boy and girl that were the exact same age but weren't twins. That way scientists could tell which effects were due to sharing a womb or sharing a house.

Obviously these are very, very rare cases. So we'd never have enough examples to be sure. For now we'll just have to say that we're still not sure what the effects of having an opposite sex twin are.

Jackie Benjamin, Stanford University

Could your twin have affected your development?

Finger length ratios and handedness are affected by testosterone