Twins and handedness


always occurs at a lower frequency than right-handedness. Generally, left-handedness is found in 10.6% of the overall population. Some studies have reported that left-handedness is more common in twins than in singletons, occurring in 21% of people who are twins. However other studies did not find a higher left-handedness prevalence in twins compared to singletons.

Monozygotic (MZ; identical) twins

The frequency of pairs of monozygotic twins in which only one twin is left-handed is about 21% and in which both twins are left-handed is less than 4%.

Left-handedness more prevalent in identical twins

The reasons for this have long been debated. Although there are many theories, such as cerebral symmetry, the reason has not been conclusively proven.

Dizygotic (DZ; fraternal) twins

The frequency of right-handed and left-handed pairs of dizygotic twins is about 23%, while twins with both individuals displaying left-handedness is less than 4%. There is no difference in the handedness frequency between MZ and DZ twins.

Chances of handedness

If the parents are both right-handed in DZ and MZ twins, there is a 21% chance of one being left-handed. If one parent is left handed in DZ and MZ twins, there is a 57% chance of one being left-handed. If both parents are left-handed, it is almost certain one twin will be left-handed.

Cross-dominance in twins

19% of twins are cross-dominant. This is the same for both DZ and MZ. Cross-dominance is when a dominant eye and dominant hand are different.

Monozygotic dichorionic twins

An early twinning event which happens before four days post-fertilization causes monozygotic dichorionic twins. MZDC twins are born in two different chorion sacs. The frequency of left-handedness in MZDC twins is 22%.

Monozygotic monochorionic twins

A later twinning event which occurs after four days postfertilization causes monozygotic monochorionic twins. MZMC twins are born in the same chorionic membrane. The data of frequencies of left-handedness in MZMC was 23%. There was not a large difference in MZDC and MZMC twins.

Mirror imaging

It is believed that this correlation between handedness in twins mirror imaging is due to MZ twins sharing the same placental blood supply, and being surrounded by the same chorionic membrane. The zygosity and chorion type do not influence twins' handedness.

Other factors of handedness