left  handed
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). 10 weeks after gestation is in the first trimester of pregnancy and probably the earliest time point in which arm movements can be investigated as before that time point, the fetus does not show many arm movements. The scientists found that the fetuses had a strong and statistically significant preference to move their right arms more than their left arms. In fact, 85% of the fetuses showed more right arm than left arm movements, which is close to the percentages in adults. Here, 89.4% are right-handers.

In addition to arm movements, other studies investigated side preferences in thumb sucking, a behavior that fetuses show that involves the hands more than just arm movements. For example, one real-time ultrasound on observe thumb-sucking in 224 fetuses found a clear preference for sucking the right thumb (Hepper et al., 1990). Overall, only 12 out of 224 fetuses (5.4%) preferred to suck on their left thumb, while 94.6% preferred the right thumb. Importantly, the researchers repeatedly recorded ultrasound from a small number of fetuses throughout pregnancy. They could show that individual preferences did not change throughout pregnancy - just like left- and right-handedness almost never changes in adults.

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. This is a rate of 93.3% that shows that in most people, left- and right-handedness is already determined long before they are even born. Only in 6.7% of people, handedness later in life was not predicted by thumb sucking preference as a fetus. This shows that later environmental factors could also influence left-handedness to some extent (see my blog post here).
Hepper PG, McCartney GR, Shannon EA. Lateralised behaviour in first trimester human foetuses. Neuropsychologia. 1998;36(6):531‐534.
Hepper PG, Shahidullah S, White R. Origins of fetal handedness. Nature. 1990;347(6292):431.
Hepper PG, Wells DL, Lynch C. Prenatal thumb sucking is related to postnatal handedness. Neuropsychologia. 2005;43(3):313‐315.