1. Fetuses Move the Right Arm More Before Birth
A study using ultrasound to examine 72 fetuses at 10 weeks of pregnancy reported that 85% moved their right arm more than their left [1]. A similar bias was found in thumb sucking: among 224 fetuses, 94.6% preferred their right thumb, while only 12 (5.4%) preferred their left [2]. Does this prenatal bias continue after birth? In a study comparing 75 people, the hand used for thumb sucking in the womb matched their dominant hand at ages 10 to 12 in about 90% of cases [3]. Secondary reports note that all 60 fetuses who preferred the right side were right-handed at ages 10 to 12 [3].
Researchers have also investigated where this bias comes from. Examining the spinal cords from the neck to the chest of five fetuses revealed differences in how genes worked on the left and right sides. Because arm movement bias exists before the brain and spinal cord fully connect, the spinal cord may be the starting point [4]. However, since this was based on only five results, it is not yet a confirmed cause.
2. Infants Begin Reaching with One Hand Around 6 Months
What about handedness after birth? In a study observing 38 infants monthly from 6 to 14 months, 15 (39%) showed a bias toward the right hand. The remaining 23 (61%) showed no preference for either hand throughout infancy [5]. Another study of 90 infants at similar ages found that a preference for the hand used to reach for and grab objects appeared around 6 months [6]. However, this does not mean handedness was fixed; it simply means a tendency to use one side appeared first.
3. Preference for the Hand That Holds and Manipulates Objects Appears Later
The preference for the hand used to hold and manipulate objects appears later than the hand used to reach. In the study of 90 infants, a statistically significant bias only emerged at about 11 months, and many children still had not fixed their manipulation hand by 14 months [6]. By the toddler years, ages 18 to 24, 76% showed a right-hand bias and 21% a left-hand bias. Children who had no preference in infancy also showed a bias by this age [5]. Handedness seems to solidify in stages, moving from the reaching hand to the manipulating hand.
4. Genetics Explains About One-Quarter of Handedness Differences
An analysis of about 54,000 twins and their siblings found that genetics explains about one-quarter (23.64%) of the variation in handedness, with the rest thought to be due to individual environments [7]. It is important to note that this percentage refers to how much of the variation in a group can be explained by genetics. It does not mean that one-quarter of an individual’s handedness is determined by genes. Additionally, an analysis of genetic data from about 500,000 people in the UK estimated the heritability from genetic differences (SNPs) to be about 4.35%, which is lower than the 20–25% found in twin studies [8]. This suggests that a single "left-handed gene" cannot explain handedness.
5. Birth Weight and Breastfeeding: How Environment After Birth Relates to Handedness
In the study of about 500,000 people in the UK, 9.3% were left-handed. Several factors, such as birth weight and whether the person was breastfed, were associated with handedness, but the differences were small and had little power to predict handedness [8]. Just because factors are associated does not mean they are the cause.
6. How to Investigate Your Own Handedness
Things you can do today: Think about which hand you use for writing, throwing a ball, or using scissors. Is it the same hand for all actions? Some people may switch hands depending on the task. If there is a baby nearby, place a toy in front of their body and observe which hand they use to take it several times. The key is not to judge by one attempt, but to observe on different days. Do not force the baby’s hand to move. If you want to learn more, the cited studies can be found in PubMed or PMC paper searches using terms like "handedness infant" [5].