Twinning is winning – mothers of twins live longer, raise larger families

By Ed Yong | May 10, 2011 7:00 pm

For every hundred babies born in Europe, one or two of them are quickly followed by a twin. From a cold evolutionary point of view, twins look like a win for their mother. History, after all, is written by individuals who are best at passing on their genes, so having more children at once seems like a good strategy. So why are twins so rare?

The standard answer is that giving birth to twins, and raising them, is difficult. Complications during pregnancy and delivery could kill both mother and children. In a classic 1990 paper, a bird specialist called David Anderson expanded on this idea. He suggested that twins are the result of an evolutionary bet-hedging strategy gone wrong.

Anderson knew that birds will commonly lay several eggs as insurance, to make sure that they get at least one strong chick. After hatching, the parents (or the strongest chick) will kill the weaker babies. In humans, a similar (but less brutal) competition happens in the womb. Mothers will often produce several eggs, with the others acting as insurance in case the first doesn’t make it. If two eggs are fertilised, one embryo is often lost – this is why many more twins are conceived than are born, the so-called “vanishing twin” effect. If this process fails, you get twins. That’s a problem, because having twins exacts a physical toll upon the mother.

According to this fairly bleak view, mums with twins have taken an evolutionary gamble that has backfired. But Shannen Robson and Ken Smith don’t agree. In a new study, the two scientists from the University of Utah have painted the birth of twins in a more positive light. To them, the very fact that some mothers can bring twins to term is a sign that they are strong and fit. The presence of twins singles out mums who can bear the extra cost of having twins.

Robson and Smith used the Utah Population Database, which collects the family trees of people who migrated to Utah in the early 1800s and their descendants. It’s one of the most comprehensive sets of family records in the world, and includes data on over 1.6 million people, right up to the 1970s. From these records, the duo pulled out 4,603 women, all of whom lived in the 19th century, survived till at least the age of 50, and had twins. They compared these women to 54,183 mums of similar characteristics who never had twins.

The duo found that women who gave birth to twins “outperformed” their peers who only ever had one child at a time. On average, they lived longer after menopause. They gave birth more frequently, and over the course of their lives, they raised two more children. They had children later on into the lives, and over a longer part of it. These advantages held even after Robson and Smith adjusted their figures to account for things like the women’s age, when they gave birth to their first child, whether their husbands or children died, and their religious affiliations.

The Utah records contradict the idea that twins harm the health of the mother. Instead, it was quite the opposite. The twin-bearers lived longer and raised more children. They even tended to have more singletons than other mums, which means that even excluding their twins, they would still have ended up with larger families.

This isn’t always the case. In 1998, Virpi Lummaa used historical records from Finland to show that mothers did better if they had twins on islands where food was reliable and abundant. However, on the mainland, where crop failures and famines were more common, women who stuck to singletons fared better. This suggests that benefits and costs of having twins depend on one’s environment.

Nonetheless, other scientists have found similar trends to Robson and Smith in other places and times, from 18th-19th century Germany to 18th-20th century Norway to modern Gambia. Others have found that mothers of twins are taller and in better health than those of singletons. However, all of these studies were based on very small numbers of records, no more than 250. By contrast, Robson and Smith’s conclusions are based on over 18 times as many twin mothers.

This doesn’t mean that the act of having twins makes women more fertile or increases their lifespan. Rather, it means that women who are more fertile and long-lived are more likely to “successfully incur the cost of twins.” In this light, twinning isn’t the failed bet-hedging strategy proposed by Anderson. Instead, it’s an opportunistic strategy – an evolutionary ace card, played by mothers in good condition, when times are plentiful.

Reference: Robson & Smith. 2011. Twinning in humans: maternal heterogeneity in reproduction and survival. Proc Roy Soc B

More on human reproduction:

Did conflict between old and young women drive origin of menopause?

The heavy cost of having children


Comments (12)

  1. Jason Dick

    Seems to me to be far, far too rare a situation to be even remotely likely to be adaptive.

  2. QoB

    Did the study control for any differences between fraternal and identical twins?

    AFAIK, a woman’s height is positively correlated with better outcomes in multiple pregnancies (hypothesised as simply more room for the foetuses to grow into before the uterus has had enough) which would back this up.

  3. Eleanor

    A likely cost, as hinted at, is the number of women who died in the process of giving birth to twins. Unless this is recorded in the records (unlikely), they can’t know what this number is and so can’t calculate the ‘fitness’ of having twins. One woman having two kids more might be offset by two women having no kids because they’re dead. I’ll read the article when it’s out!

    For it to work as a strategy, women would have to be able to assess their condition, and based on that ‘decide’ to have twins. So it would be interesting to see if larger, healthier women were more likely to have twins than smaller and less healthy ones. (Kind of like the situation in red deer, where females are more likely to have male calves if they are in good condition, but have female calves if they are in poor condition).

  4. Bee

    It would be interesting to know if the “outperformance” in terms of family size depends on the order of the children. If the last kids were twins the larger family size is accidental rather than by wish. There’s also the question if not the effect is more psychological than physical. If your first kids are twins, a singleton pregnancy sounds like a piece of cake. I didn’t know hight matters for the outcome of a twin pregnancy. Glad nobody told me :)

  5. This article have natural selection all over the place. It shows a direct correlation of success rate of having and raising twins depending on the environment.

    So, if food was always abundant in the past we would probly have a lot more twins nowdays. Those womens who were more apt to have more babies would always had bigger familys what would carry their genes far longer then the ones with singletons.

    So instead of a 1% rate of twins birth we could have 10%. By the way, theres a city in Brazil with the name of “Cândido Godói” this city has really above normal birth rate of twins, nobody knows exactly why.

  6. Michelle

    What about mothers that became pregnant and carried 3 or more fetuses to term. Granted, this is much more likely to happen in this day and age with fertility treatments, I am sure that there were women who conceived and delivered triplets or more.

  7. positively positive

    Utah isn’t exactly a typical state, so how generalizable are the results. Also, with fraternal twins, the likelihood increases with the number of previously pregnancies, so not exactly predictive.

  8. ariadeer

    Very interesting. Here’s my story. I had three pregnancies that didn’t make it. Then I became pregnant (naturally) with fraternal twins. I went on to have two more children. My last one at 37. Both my great-grandmothers had three sets of twins each. And, one of those great-grandmoms lived to be 101. I don’t know how long the other lived but one of her non-twin daughters is still alive at 94. And, my parents are 78. And, nearly all their siblings are still alive into their eighties. The one thing that doesn’t apply to me in this article is the fact that the females in my family tend to be petite. I’m 5’2. And, my twins were two days late at 7lbs. 10 oz. and 7lbs. 4oz. I would like to know if the genes of the twins are hardy because they were the fighters in the womb trying to make it to birth.

  9. I work with womb twin survivors – the sole survivors of “vanishing twin” pregnancies, or people whose twin died in the womb or around birth. Some of then have a problem with their mothers, and it seems that they blame their mothers for not being strong enough to carry both twins to birth and keep the twin pair intact. Bringing up twins or multiples is not for the faint-hearted! Only very tough mothers and fathers can do that. Its amazing to think that for every pair of twins who get to birth alive, there are TEN womb twin survivors!

  10. An interesting theory. I am the Mother of 2 sets of Twins ( Eldest and Youngest) Eldest are boys Born when I was 21 and Youngest a boy and Girl Born when I was 30. I also had 2 single Births in between. I am now 71 and I have had good health most of my life, never smoked, and my Husband is 91 years young. I truly believe that our Family has kept my Husband young in outlook and he was 50 when 1st Twins were Born. ( He is also a D Day Veteran. There were Twins on both sides of our Families so it was almost inevitable that we would have Twins. Hope you found this comment useful Thank You.

  11. Mintery

    Perhaps it should have been mentioned that this “strong sturdy twin mother” study only considered mothers who conceived twins naturally—NOT those who used fertility treatments of any kind.

    I know 8 people who have “medicine” twins (meaning they took clomid or used IUI or IVF) and have one friend who had natural spontaneous twins. All the medicine twins are fraternal and the natural twins are identical. It seems identicals are the only rare twins left in society. They are always a fluke of nature and not forced. Fraternals are not really rare at all any more.

    Interesting article!

  12. Lisa

    I have fraternal (dizygotic) twins via fertility treatments and I am very fascinated by how twins occur. My local parents of multiples group has a wide variety of twin types.

    From what I have read monozygotic (identical) twins are a very random event that can happen to anyone. There does not seem to be a proven genetic link. However, there is some anecdotal evidence that IVF treatments (particularly those which use assisted hatching) increase the rate of monozygotic twins.

    Recent research has shown that dizygotic multiples (conceived without fertility treatments) does have a genetic link. Some women are more susceptible to the effect of the LH (Luteinizing hormone) and (if I recall correctly) FSH (Follicle-stimulating hormone). This means that they are more likely to produce and release more than one mature egg each cycle. So if a woman’s maternal line has many sets of multiples, this is what is happening.

    I have not seen any evidence that males are able to pass on a genetic likelihood for twins. It’s all about the egg.


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