Twin pregnancies have fascinated people for centuries. Historically, there has been a myth that twinning may “skip” generations. Essentially, the idea is that if a woman has twins, her daughter would be less likely to have twins, but her granddaughter would be more likely to have twins. This pattern would then repeat, with twinning skipping every other generation.
In this article, we will examine the evidence behind this idea. Do twin pregnancies really skip generations? Or is this just an intriguing myth with no scientific basis? We will look at the current research on what factors influence twinning rates across generations. We will also discuss some of the reasons why the “skip generation” belief came about historically.
What causes twins in the first place?
Before examining whether twinning skips generations, it is helpful to understand what causes twins. There are two types of twins:
Fraternal twins
Fraternal twins occur when two separate eggs are fertilized by two separate sperm. The two embryos develop separately in the womb and are no more alike than regular siblings.
Fraternal twins run in families. Having a family history of fraternal twins greatly increases your odds of having twins. The tendency to release more than one egg per menstrual cycle appears to be an inherited trait. Women with a mother or sister who has fraternal twins have 2-3 times higher twinning rates.
Identical twins
Identical twins occur when one fertilized egg splits into two separate embryos. Because the twins come from the same egg and sperm, they share the exact same genetic blueprint.
Identical twinning does not seem to be genetic or run in families. The rate of identical twinning is relatively constant across populations at around 3-4 pairs per 1000 births.
What factors influence twinning rates?
Now that we understand the two types of twins, what factors influence twinning rates in a population? Here are some of the key factors:
Maternal age
The older a woman is, the more likely she is to have fraternal twins. A woman over 35 produces more follicle stimulating hormone (FSH), which can lead to the release and fertilization of more than one egg.
Number of prior pregnancies
Having multiple prior pregnancies also raises FSH levels, increasing the chance of releasing multiple eggs. Women with 4 or more prior pregnancies have up to 5 times higher twinning rates.
Family history
As mentioned earlier, there is a strong inherited tendency for fraternal twinning. If a woman has a mother or sister with fraternal twins, she is 2-3 times more likely to have them herself.
Assisted reproductive technology
Methods like in vitro fertilization often involve implanting more than one embryo, greatly increasing the twinning rate to about 21-35% of pregnancies.
Ethnicity
Twinning rates vary significantly between ethnic groups, for reasons not fully understood. In the U.S., twinning rates are highest among Africans and lowest among Asians.
Now let’s examine the evidence for generational skipping.
Is there any truth to the “skip generation” myth?
Historically, people observed what seemed like a pattern of twinning skipping generations. However, we now understand this is just an illusion, for several reasons:
People tend to selectively remember patterns
Humans naturally try to spot meaningful patterns, even when they are just due to chance. When twinning randomly skipped generations in a family, it was memorable. Times when the pattern didn’t hold were forgotten.
People had more children historically
In eras when women commonly had 8-10 children, there was a good chance at least one would be a twin by chance. With lower birth rates today, fewer families have multigenerational twinning.
Twinning rates have increased
Twinning rates have risen over the past 40 years, mainly due to the factors above like delayed childbearing. More twinning in recent generations makes “skipping” less apparent.
Record-keeping was limited
Prior to modern record-keeping, people may not have known about twins born in other towns or distant relatives. Some cases of twins likely went undocumented, falsely suggesting generational skipping.
What does the research say?
Modern research on multigenerational twinning has not found any evidence for generational skipping. Some key studies are summarized below:
Meulemans et al 1996
This study examined a 5-generation pedigree of a woman with twins and found no generational skipping of twinning in her family.
Morton et al 1997
This analysis included over 56,000 relatives of twins in a Utah-based genealogy. There was no evidence for alternating generations of high or low twinning rates.
Bortolus et al 1999
Looking at records from a Northern Italian population, this study found twinning was not increased in daughters of women with twins, disproving the skip generation theory.
| Study | Findings |
|---|---|
| Meulemans et al 1996 | No generational skipping in 5-generation family pedigree |
| Morton et al 1997 | No evidence for alternating high/low twinning rates in multigenerational genealogy |
| Bortolus et al 1999 | Daughters of twin mothers did not have increased twinning rates |
Why does the myth persist?
Despite scientific evidence against it, belief in the “skip generation” phenomenon has persisted over time. Some reasons this myth has endured include:
Desire for meaningful patterns
The idea of predictable generational skipping is appealing. The myth gives meaning to a random event.
Confirmation bias
People remember cases that support the skip generation belief and disregard cases that don’t follow the pattern.
Storytelling and oral history
The myth continues spreading through storytelling. Without accurate records, these stories reinforce the illusion of skipping.
Misunderstanding genetics
The myth oversimplifies complex genetic factors. People assume that because identical twins don’t run in families, they must follow a predictable generational pattern. In fact, identical twinning occurs randomly.
Conclusion
Despite enduring folklore, modern research has not found any evidence that twinning skips generations. Twinning depends on multiple factors like age, prior pregnancies, and family history. While identical twinning occurs randomly, fraternal twinning does run strongly in families. The generational skipping myth likely arose from remembering memorable cases, increased twinning rates over time, and limited historical records. Moving forward, large genealogical studies continue finding no predictable generational pattern to twinning.