Twins and handedness
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 21% of people who are twins.
- 1 Monozygotic (MZ; identical) twins
- 2 Why is left handedness more prevalent in identical twins?
- 3 Fraternal twins
- 4 Chances of handedness
- 5 Cross-dominance in twins
- 6 Monozygotic dichorionic twins
- 7 Monozygotic monochorionic twins
- 8 Mirror imaging
- 9 Other factors of handedness
- 10 See also
- 11 References
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%.
Why is 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. 
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.
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 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 your dominant eye and dominant hand are different and if one twin is both right-handed and right-eyed, they have most likely been forced to use their non-dominant hand by a school teacher or parent.
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.
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.
Other factors of handedness
- Family history of left-handedness 
- Birth stress- forces and injuries of baby's head 
- Gestational age 
- Sex 
- Hair whorl 
- Social pressure
- Derom, C., Thiery, E., Vlietinck, R., Loos, R., and Derom, R. 1996. Handedness in Twins According to Zygosity and Chorion Type : A Preliminary Report. Behavior Genetics 26 : 407-408.
- Rife, D.C., 1939. Handedness, with special reference to twins. Genetics 25: 178-186
- Churchill, A. J., Igna, E., and Senf, R. 1962. The Association of Position at Birth and Handedness. Pediatrics 29 : 307-309
- Shimizu, A., and Endo, M. 1983. Handedness and familial sinstrality in a Japanese student population. Cortex 19: 265-272
- Ooki, S. 2006. Nongenetic Factors Associated with Human Handedness and Footedness in Japanese Twin Children. Environmental Health and Preventive Medicine. 11: 304-312
- Klar, J. S. A. 2003. Human Handedness and Scalp Hair-Whorl Direction Develop From a Common Genetic Mechanism. Genetics 165: 269-276