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Related to the handedness question: Can you discuss some of the new research that indicates left-handedness appears to be associated with in utero levels of testosterone and genetics?

-A curious adult from Texas

January 25, 2007

There are many theories for why 12% of the population is left handed. In a previous answer I discussed the idea that something in the womb happens to trigger it. And that some people are more likely to respond to that something because of their genetics.

One theory is that the trigger is testosterone. Scientists considered testosterone as a potential trigger mostly because men are more likely to be lefties than women.

And men are exposed to more testosterone as a fetus. To become a boy, a fetus gets a blast of testosterone early on that turns it into a boy.

So maybe boys are more often left handed because of this extra testosterone. And maybe girls who get an extra dose of testosterone have a greater chance of ending up left handed as well.

To date the studies have been mixed on whether testosterone leads to being left handed. Some studies suggest there is a link while others do not.

Part of the reason for these mixed results has to do with how testosterone levels are measured. It is really hard to directly measure the testosterone levels of a 12-week-old fetus. So scientists started looking for an indirect way to measure it.

One interesting measure they use is finger lengths. Back in 1888, Baker noted that men and women are different in terms of their finger lengths.

Women tend to have index and ring fingers that are about the same length. Men on the other hand (pardon the pun) tend to have a ring finger that is longer than their index finger.

In other words, on average, males have a longer ring finger compared to their index finger than do females. Scientists have proposed that the ratio of index finger to the ring finger reflects how much testosterone the fetus is exposed to.

So the idea would be that left-handed folks would have shorter index fingers compared to their ring fingers. Despite some early studies, this hasn't held up particularly well.

On average, left handed people don't seem to have lower index finger to ring finger ratios. A couple of studies have shown that people with more "male" index fingers tend to do better on certain left handed skill tests. But they didn't show that lefties have more male-like fingers.

So does that mean that finger ratio is not a good measure of in utero testosterone levels? Not necessarily.

It may be that handedness isn't linked to this trait. Or that the differences are just too subtle to accurately measure.

Another major problem is that the differences between ethnic groups can be higher than the differences between the sexes. For example, people of African descent tend to have lower finger ratios overall. So a study that is racially diverse may give different results than one that is of a single ethnic group.

Another indirect measure has been to look at the androgen receptor (AR) gene. How much testosterone we make is partly determined by the version of the AR gene we have.

What one group found was that if someone has an AR gene associated with higher levels of testosterone, they are more likely to be left handed. Interestingly these folks tended to have more male-like finger ratios.

I want to caution, though, that this was a single study that will need to be further tested.

Finally, someone went to the trouble of measuring fetal testosterone levels. Unfortunately it wasn't a study that looked at handedness but instead looked at the finger ratios.

What they found was that the ratio was dependent on how much testosterone was present compared with how much estrogen was present. This can't be done easily for a lot reasons including the risks of amniocentesis.

So where are we on handedness? I think we can safely say that prenatal levels of testosterone might influence whether someone is left handed. To really nail it down, we'll need more studies like the last one where testosterone levels were directly measured.

Incidentally, scientists have gone a bit wild using the finger ratio to study all sorts of traits and behaviors. Like homosexuality, depression, autism, athletic ability, face shape, semen quality, and on and on. Not surprisingly, these studies are mixed.

By Dr. Barry Starr

Handedness may depend on finger lengths!

Differences in finger lengths between ethnic groups complicates handedness studies.