Left-handedness always occurs at a lower frequency than right-handedness. Generally, left-handedness is found within 10.8% of the overall population. However, left-handedness is more common in twins than in single individuals, occurring in 17% of people who are twins.
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Monozygotic (MZ; identical) twins
The frequency of pairs of MZ twins exactly one twin left-handed is about 21%, and, with both twins left-handed, less than 4%.
Fraternal twins
The frequency of right-handed and left-handed pairs of DZ twins is about 21%, 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.
Monozygotic dichorionic twins
An early twinning event which happens before 4 days post-fertilization causes monozygotic dichorionic (MZDC) 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 4 days postfertilization causes monozygotic monochorionic (MZMC) twins. MZMC twins are born in the same chorionic membrane. The data of frequencies of left-handedness in MZMC was 23%. There was no big 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. It is hard to think that discordant handedness in MZ twins demonstrated mirror imaging.