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As a matter of clear and simple fact, dizygotic twins are much more likely to differ in handedness than monozygotic. So, if a difference in handedness means an embryonic "splitting" that may have happened fairly late in embryonic development, then we simply must conclude that dizygotic twins are more like to have experienced such events than the monozygotics! Another thing about it - no one, ever, no twin, no singleton, no one, ever, has been found with the normal asymmetries of brain structure or function completely reversed. Not one. Not ever. The normal left-right differences in brain structure and function are very basic and important facts of normal development. I collected handedness data from over 800 three-generation twin families. I was able to use the results from 773 families, over 10,000 people. The results are straightforward: Twins do have a higher frequency of lefthandedness than the general population, which is mostly single-born. Monozygotic twins and dizygotic twins have the same higher frequency. Monozygotic twins do not have a higher frequency than dizygotic twins. There is no difference in frequency of lefthandedness between monozygotic and dizygotic twins. No difference. The monozygotic twinning process, whatever it may turn out to be, is not in any demonstrable way related at all to what Spemann did to the newts. The monozygotic twinning process does not differ from the dizygotic process in any of the causes of lefthandedness. The twins do not have a significantly higher frequency of lefthandedness than their single-born siblings. So, nothing about being twins causes lefthandness. Twin embryogenesis does not increase the frequency of lefthandedness over that of the singleborn siblings of twins. Nothing about twin pregnancy causes lefthandedness. Nothing about who spent more time on top or on whichever side of the womb. Nothing about the events of twinning causes lefthandedness. So where does the excess of lefthandedness among twins come from? The single-born siblings of the twins also showed a substantially higher frequency of lefthandedness than the general population. There was no difference between the siblings of monozygotic twins and the siblings of the dizygotic twins. The twins do have very slightly more lefthandedness than their singleton siblings do. The difference is not statistically significant - which means it could be due entirely to chance in selecting that particular sample from a population in which there was truly zero difference. That means we really have no good reason to believe there is any real difference between twins and their siblings in frequency of lefthandedness. But there is something interesting about it. The excess in second-born twins was more than enough to explain the difference between twins and their single-born siblings. When those twin-family handedness data were collected in 1975-78, more than half of all twin pregnancies were discovered to be twins only after one baby had been delivered and somebody realized that Mama was still pregnant. There was very commonly a delay between the delivery of the first twin and the delivery of the second, often a delay of several minutes. During that delay, the second twin was subject to a greatly reduced flow of maternal blood - reduced supplies of food and oxygen, reduced waste removal - suffocation in a word. The right side of the baby's brain normally has a substantially larger blood flow. The left hemisphere is more vulnerable to the effects of the temporary asphyxiation. So, a small excess of lefthandedness in twins relative to their single-born twins, although not statistically significant, may be a biologically real consequence of slightly-reduced left-hemisphere functionality due to temporary asphyxiation of second-born twins. Now that ultrasound examination is routine in North American and Western European obstetric practice, that difference should be eliminated. Twins born since the early- to mid-1980s might not show such a difference, but I don't think anybody has tested it properly. |