International Left-Handers Day
International Left-Handers Day is held annually every August 13
- Handedness is often defined by one’s writing hand, as it is fairly common for people to prefer to do a particular task with a particular hand. It reflects the wiring of the individual brain
- Around 10% of population is left-handed.
- Very young children often use both hands equally. Hand preference in the early years seems to rely on which hand is closer to the desired object; for example, a toddler may reach for a toy on their left side with their left hand because of convenience, regardless of future hand preference.
- Most children have a preference for using one hand or the other by the age of about 18 months, and are definitely right or left-handed by about the age of three.
- However, a recent UK study of unborn babies found that handedness might develop in utero. About nine out of 10 unborn babies preferred to suck their right (rather than their left) thumb, and this hand preference was borne out later in life.
- In the late 20th century, left-handedness became less stigmatised, and in many countries, particularly the Western world, left-handed children were no longer forced to switch to their right hand.
Potential causes
Info
Just why one in 10 people favour their left hand is a mystery. A straightforward genetic link hasn’t been proven, and it is possible for two right-handed parents to have a left-handed child.
Genetics
- Handedness displays a complex inheritance pattern
- If both parents of a child are left-handed, there is a 26% chance of that child being left-handed.
- Heredity of handedness is roughly 24% (Medland et al. 2006).
- McManus et al. conclude that handedness is polygenic and estimate that at least 40 loci contribute to the trait.
- Brandler et al. performed a genome-wide association study for a measure of relative hand skill and found that genes involved in the determination of left-right asymmetry in the body play a key role in handedness.
- Wiberg et al. performed a genome-wide association study and found that handedness was significantly associated with four loci, three of them in genes encoding proteins involved in brain development.
Gender
- Slightly more boys than girls are left-handed. This suggests that the male hormone testosterone has an influence on handedness.
- According to a meta-analysis of 144 studies, totalling 1,787,629 participants, the best estimate for the male to female odds ratio was 1.23, indicating that men are 23% more likely to be left-handed.
Foetus development
- Some research suggests that handedness can be influenced through the environment more than genetics. Environmental factors in the womb (including the exposure to hormones) may influence which hand we favour.
- Four studies have indicated that individuals who have had in-utero exposure to diethylstilbestrol (a synthetic estrogen based medication used between 1940 and 1971) were more likely to be left-handed over the clinical control group.
- Diethylstilbestrol animal studies “suggest that estrogen affects the developing brain, including the part that governs sexual behaviour and right and left dominance”.