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Thursday, May 10, 2012

Why are Asian women less likely to have twins?

In our cell biology class on placenta and human reproduction today, the professor talked a bit about twinning. According to him, the rate of monozygotic (identical) twinning is fairly constant across populations, but the rate of dizygotic (fraternal) twinning varies considerably, depending on maternal characteristics. In order to have dizygotic twins, a woman must at least ovulate two eggs, at least two of which must be fertilized and be implanted in the woman's womb. The following are true of the rate of dizygotic twinning:

1. More likely among overweight women.
2. More likely among women in their 30s.
3. More likely among women of African ancestry, and least likely among Asians. Whites are in between. According to wikipedia, the twinning rate of the Yoruba people in Africa is 40 per thousand. That of the Japanese is 6 per thousand.


#1 makes perfect evolutionary sense. In the evolutionary environment, being overweight was a signal that nutrition is abundant. It would have been evolutionarily advantageous to have twins when food was aplenty. Similarly, overweight children tend to enter puberty at an earlier age. In contrast, severe dieting often delays puberty in teenagers and often causes women to skip an ovultion. The human body is designed to reproduce more in times of plenty, and reproduce less in times of famine.

#2 is paradoxical. I can't come up with an evolutionary justification. Could it be that women in their 30s tend to have more dizygotic twins primarily because they are more likely to use In Vitro Fertilization to become pregnant than women in their 20s? In IVF, it is a routine practice to introduce two or more fertilized eggs into the mother, because the natural failure rate for implantation onto the womb is very high (majority of fertilized embyos do not successfully implant).

#3 is partly driven by the tendency of Asians to be thinner. But there was almost certainly selection pressure against twinning in Asians and (to a lesser extent) in White people, given the humongous variation in the rate of twinning and the widespread distribution of the gap. All human populations outside of Africa descend from a small share of prehistoric humans that left Africa - our species' birthplace.  In many ways, Africans are our species genetic "default."  Since East Asia and Europe were relatively inhospitable, cold, dry places during the Ice Age when humans first populated them, it may have been particularly disadvantageous to attempt to care for two babies at the same time.   Perhaps for the same reason, Whites and Asians hit pubterty about 2 years after people of African ancestry do.

3 comments:

  1. Interesting, I think Jensen and particularly Rushton have noted this difference in twinning rates.

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    1. Rushton's theory is that Africans are more r selected (fave lots of offspring, but give low parental care to each) and Asians are more K selected (have few offspring, but dedicate more care to each). The lower rate of twinning among Asians is consistent with Rushton's theory, but it could just be that in the ancestral environment of East Asians, there simply weren't enough resources to support many children.

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  2. I have twins and an interest in the topic due to recurrent miscarriage. There is a point around 35 at which twins peak, because the miscarriage rate is still relatively low and multiple ovulation becomes more common. Above 35, miscarriage is common enough that pregnancies that start as twins don't remain twins. I suspect that multiple ovulation increasing prior to menopause naturally compensates for an increased miscarriage rate.

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