You Suck: Thumbs in the womb as predictors of handedness

I actively look for articles and studies on handedness but they’re not always easy to find.  Sometimes I only run across them by accident, like this item from June 2020 on handedness in the womb.  It addresses the “nature or nurture” argument about handedness:

Does Left-Handedness Develop Before or After Birth?

About 10.6% of humans are left-handed (Papadatou-Pastou et al., 2020). One of the longstanding questions in scientific research on left-handedness is, at which point in life it actually develops.

One commonly held idea is that it is possible to know for certain whether a child is left-handed or not once he or she starts writing. However, scientific studies show that left-handedness actually develops much earlier to in primary school. In fact, it actually develops before we are even born.

Scientists have investigated left-handedness in unborn babies using real-time ultrasound recording in order to track the movements of their arms and hands in the womb.

For example, one study analyzed real-time ultrasound recordings of 72 fetuses 10 weeks after gestation, focusing on left and right arm movements (Hepper et al., 1998).

[. . .]

The big question about these studies on arm movements and thumb sucking is, whether these behaviors actually predict left- and right-handedness later in life or are unrelated. To answer this question, a longitudinal study tested handedness in 75 children aged between 10 and 12 years in which thumb sucking preferences had been examined using real-time ultrasound recording when they were still fetuses (Hepper et al., 2005).

The results were quite impressive.

Overall, 60 of the children were right-handed as fetuses and all 60 were also right-handed at ages 10 to 12. Thus, a preference to suck the right thumb as a fetus is highly predictive of right-handedness in later life. The remaining 15 children preferred to suck the left thumb as fetuses. In this group, 10 children were left-handers at ages 10 to 12, and 5 were right-handers.

Thus, thumb-sucking preferences as fetus predicted handedness as a child in 70 out of 75 children correctly.

The emphasis at the end is mine.

Once again, correlation does not prove causation, but it does prove this should be studied more.  Leave lefty kids alone until the science is in.  And even then, leave them alone.


  1. Bruce says

    All humans are raised in cultures dominated by teachers etc who are mostly right handed. Thus, any statistic that, say, 10% of people are left handed is really measuring the combined effects of nature and of nurture under these circumstances.
    I doubt any society will ever try the experiment of taking randomly selected kids away from their families at birth, to be raised exclusively in environments with only left handed people. It would be hard to find even a natural experiment of kids raised by lefties who send the kid to a school with only lefty students and only lefty teachers.
    My point is that the cited data suggests that the natural percent of lefties in equal environments would not be 10%, but over 20%.
    So half of all born-lefties likely get trained or persuaded into thinking they are just awkward right-handed people, and never think they could have been a lefty.
    Even the best-intended surveys will be missing a huge amount of the story.
    I wonder what other aspects of human nature also fall under such invasive yet invisible pressure? If the pressure on lefties, instead of cutting 21% down to 10.2%, perhaps cut it down from say 3% to 1%, then we could speculate that humanity might never notice or admit that being left-handed was even a possible question.
    What other aspects of life might be accidentally hidden and sublimated without any of us even knowing that a question had existed? Fascinating.

  2. kestrel says

    That is really interesting. Many years ago I saw a program on PBS where scientists were looking at right- or left-handedness in mice. The mice had been trained to climb a stick and at the top there was a tube where they could get out a piece of food while still hanging on to the stick, and the idea was to look at which paw they preferred for grabbing the food. Some of the mice used the left and some used the right. That always stayed with me, that **animals** could have handedness. That would be hard to study of course but there are probably bears, or coyotes, that prefer reaching or leaping to the left rather than the right.

    But yes, absolutely. As an ambi, please, let children use whatever hand they wish. Both work great, I can assure you.