How Do We Explain the Birth of Fraternal Twins?
Evolutionary theories may explain fraternal twinning.
Posted Jul 31, 2018
Fraternal Twins: Explaining Their Presence
The birth of twins in all human populations is puzzling because the female uterus is designed to carry just one fetus. Multiple birth pregnancies have higher frequencies of serious physical and health hazards for both mothers (e.g., preeclampsia, delivery complications) and for infants (e.g., immature lung development, growth retardation), than do singleton pregnancies. However, evolutionary-based explanations of fraternal twinning offer insights into this relatively rare event.
Twinning rates differ across human groups. The world’s highest rate is in Benin at 27.9/1,000 and the world’s lowest rate is in Vietnam at 6.2/1,000. These figures combine identical and fraternal twins, but because identical twinning occurs fairly uniformly across human groups (3/1,000 to 4/1,000 births), variation in the fraternal births largely explains the population differences.
It is well known that relatively older women, age 35 years and beyond, are more likely to conceive fraternal twin twins because of their increased chances for multiple ovulation. This event has been thought of by many as a reproductive mistake since most females release just one egg at a time. However, some evolutionary theories have revised thinking on this topic. One of the first theories rests on the “insurance ova hypothesis,” which states that while selection does not favor multiple births it does favor multiple conception attempts in each cycle. Releasing more than one egg increases the chance of producing a newborn, leading to two predictions: 1) egg production should be higher than the rate of birth, which it appears to do, and 2) multiple ovulation offers insurance against genetic problems because the embryos have different genotypes.
Anderson’s reasoning was extended by other investigators with the idea that fraternal twinning may be a by-product of selection for multiple ovulation that sometimes eventuates in an error, namely the birth of twins. As such, producing more than one egg reduces the risks of embryo defects and the risks of conception failure.
In the late 1990s, when I was writing my first book, Entwined Lives, it occurred to me that fraternal twinning might be understood at two levels. At one level, multiple ovulation could be thought of as a reproductive error, as indicated above. However, at the evolutionary level, it might signal a final effort by older women toward genetic representation in later generations, as they approach the end of their reproductive period. I was disappointed to learn that another investigator, L. Scott Forbes, had published this same conclusion in 1997.
Forbes discusses the relaxed screening hypothesis, asserting that older mothers should become less selective about their offspring quality since 1) defective offspring are better than no offspring–females with Down syndrome (which increases in frequency among infants conceived by other mothers) are fertile and having a normal child later in life may reduce the cost of a maladaptive one. Furthermore, relaxing the maternal screen increases the chance of bearing a genetically defective child, but also a normal child. This can be seen as a kind of maternal trade-off. In summary, Forbes suggests that “Twinning is not about twins. It is about avoiding pregnancy failure.”
The presence of both identical and fraternal twins, triplets, quadruplets and other higher order multiples in human populations offers important ways of studying influences on behavioral and physical traits. Their origins and presence are fascinating issues that require creative answers.
This essay has been adapted from a longer article published in the journal Twin Research and Human Genetics:
Segal, N.L. (2018). Human dizygotic twinning: Evolutionary-based explanations.Twin Research and Human Genetics, 21 (Special Issue 4), 325-329.
Other references are listed below.
Anderson, D. J. (1990). On the evolution of human brood size. Evolution, 44(2), 438-440.
Ball, H. L., & Hill, C. M. (1999). Insurance ovulation, embryo mortality and twinning. Journal of Biosocial Science, 31(2), 245-255.
Forbes, L. S. (1997). The evolutionary biology of spontaneous abortion in humans. Trends in Ecology & Evolution, 12(11), 446-450.
Forbes, S. (2005). A natural history of families. Princeton NJ: Princeton University Press.
Segal, N.L. (1999/2000). Entwined lives: Ywin and what they tell us about human behavior. NY: Dutton/Plume.
Segal, N.L. (2017). Twin mythconceptions: False beliefs, fables, ands fact about twins. San Diego: Elsevier.
Smits, J., & Monden, C. (2011). Twinning across the developing world. PLoS One, 6(9). P. e25239.