Do Males Affect Twinning Events?

Who has twins, and why, are fascinating and tantalizing questions. Most past and recent research has emphasized female contributions to twinning, especially fraternal twinning. Factors such as greater maternal height and weight, ancestry, family history and increased coital frequency have been cited as predisposing women toward having fraternal twins. But there have bene hints that males also have something to do with it. The idea for this article came to me when I was contacted by a young male who had fathered several sets of non-identical twins with several different women.

Based on studies of individual families, an early study concluded that asserted that both one-egg (identical) and two-egg (fraternal) twins were passed down on the maternal family side. However, it was speculated that the high incidence of twins among the fathers’ brothers suggested paternal transmission. Another early researcher believed that both mothers and fathers can be genetically predisposed toward conceiving identical twins, but that only mothers carried the tendency toward conceiving fraternal twins. It was also suggested that the increased twinning sometimes observed among the siblings of fathers of twins was explained by underreporting of singleton births on the paternal family side.

More Twins for Female Twins and Their Sisters?

A review of family data gathered by the Genealogical Society of the Church of Jesus Christ of Latter Day Saints, in Salt Lake City, Utah, was informative. Here, the focus was on the twins themselves. It was noted that fraternal female twins and their non-twin sisters showed increased twinning among their offspring. However, this trend was not observed among males.

A case report some years later, in the 1970s, described a case of familial twinning in which twin births, represented equally in males and females. There were six pairs born across four generations and all included a normal twin and either a macerated fetus, stillborn infant or premature infant. It was thought that superfetation—the release of a second egg in what would be considered a subsequent menstrual cycle—was operative. It was also proposed that because both parents contribute to the genetic makeup of the placenta, either the mother’s or the father’s genetic background could have caused the superfetation.

Another important development was reported several years later, in the 1980s. It seemed found that Black mothers showed a higher frequency of opposite-sex twins than Caucasian mothers. However, after adjusting for mothers’ race, Black fathers did not show higher rates of opposite-sex twins than Caucasian fathers, indicating that fraternal twinning is affected by maternal race only. Then, a study that tracked the paternal transmission of twinning in three Scottish pedigrees appeared, using data from 1800-2000. It was found that there were paternal effects for both identical and fraternal twinning, as well as twinning tendencies associated with partial male infertility.

Specific genes, gene variants and hormone levels have been linked to DZ twinning. Nick Martin, editor of Twin Research and Human Genetics (TRHG), has played a pivotal role in this area since the mid-1980s. Future studies with larger twin populations will be of importance in this regard, with additional focus on paternal effects. At present, there is a small but growing literature in this area.

Growth Factor and Twinning

A study conducted in Israel found an older father influence on twinning; however, the effect was slight and was only observed for opposite-sex twin pairs and male-male twin pairs. Explanations for why this was the case are unavailable. Subsequent research on over eight million births in the United States found that fathers in their forties were more likely to have twins of both types, relative to younger fathers. However, the higher twinning rate for fathers was still lower than that for mothers. Furthermore, paternal age had the greatest effect on twins born to Caucasian non-Hispanic fathers.

There are other clues as to how males may influence twinning. An increased level of insulin growth factor-2 (IGF-2) is associated with cell growth and division, and an individual’s IGF-2 level is partly heritable. The IGF-2 gene transmitted by fathers to their children is active, whereas the same gene transmitted by the mother is inactive. It is, therefore, possible that MZ twinning is elevated in some families by the paternally transmitted level of IGF-2.

Danish studies of semen quality and twin pregnancy are also revealing. Studies conducted in the past ten to fifteen years showed a genetic effect on plasma levels of Sertoli cell function and to sperm cell chromatin stability and morphology. (Sertoli cells play a leading role in the formation and development of the testis in fetal and early postnatal life. They also provide the environment and support for germ cells during spermatogenesis after puberty). Environmental effects, including the prenatal environment, were largest for sperm count. Other work found that the fathers of both fraternal and identical and fraternal twins had percentages of motile sperm that were 11.5% and 12.5 %, respectively, higher than those of fathers of singletons. The authors of this study were surprised that their findings applied to findings to identical twins, as well as fraternal twins. They also noted that their study prior research showing a included it is likely that this information would have been provided. It would also be of important to know if women who conceive DZ twins so do subsequently, but with other males.

Understanding Twins Essential Reads

"The Accidental Twins" and "Peter and Erik"

The Unique Development of Twin Language

It is worth noting that there is some evidence of familial identical twinning in places around the world. Specific genes linked to identical twinning have been proposed, although transmission on the paternal or maternal side has not been indicated. This constitutes a fascinating research direction for the future—it has always amazed me that we know so much about identical twins, but so little that is certain about their origins. I would also like to see increased study of males who fathered twins with different women, and (if the situations allow) follow-up study of these women to see if they conceive additional twins with different males.

Additional details on this topic can be found in my longer article, “Do males affect twinning events? A review of current findings.” Twin Research and Human Genetics, (online, DOI: https://doi.org/10.1017/thg.2021.8, journal: in press).