For most major health outcomes, twins do not face meaningfully higher risks in adulthood than people born as singletons. A large Danish study tracking twins through their entire lifespan found that cumulative risks of cancer, cardiovascular disease, and death were essentially the same as in the general population. But this reassuring headline hides some interesting wrinkles: a handful of studies flag elevated diabetes risk in elderly twins, monochorionic identical twins face pregnancy-specific complications that can leave lasting marks, and the social bond between twins may actually confer a survival advantage that singletons lack.
Mortality, Cancer, and Heart Disease
The most direct way to ask whether twins are disadvantaged is to compare their rates of the diseases that kill the most people. A population-based study using Danish twin registries did exactly that, following twins across their full lifespan and comparing them to both siblings from twin families and singletons from non-twin families. The result was strikingly clear: cumulative risks of cancer, cardiovascular disease, and death in twins did not differ from singletons, regardless of which comparison group was used.1International Journal of Epidemiology. Twinship influence on morbidity and mortality across the lifespan The researchers concluded that despite their rougher start in the womb, twinning does not lead to worse long-term health outcomes.
Cancer specifically has been examined in its own right. A study comparing cancer rates in twins to their singleton siblings found no consistent differences across cancer sites. About 9% of male twins and 18% of female twins developed a primary cancer, compared to 11% and 19% of their singleton siblings, and none of these differences reached statistical significance when individual cancer types were analyzed.2PubMed. Cancer risks in twins and singletons from twin and non-twin families
Heart disease tells a similar story. A Swedish study tested whether twins’ lower birth weight translated into higher mortality from ischemic heart disease, since a prominent hypothesis links poor fetal growth to later cardiovascular trouble. Among more than 14,000 twins, heart disease mortality was not elevated compared to the general Swedish population. For men, the risk was actually lower.3PubMed. Ischaemic heart disease and low birth weight: a test of the fetal-origins hypothesis from the Swedish Twin Registry This finding matters because twins are a natural test case for the idea that being born small programs your body for disease. If that theory held up cleanly, twins should be worse off. They aren’t.
Why Being Born Small Doesn’t Doom Twins
Twins are almost always born smaller than singletons. They share a uterus and compete for nutrients, and they tend to arrive earlier. For decades, researchers have wondered whether this makes twins a living experiment in intrauterine growth restriction, the condition in which a fetus doesn’t grow as well as it should. The short answer is that the kind of growth restriction twins experience appears to be different from the pathological version seen in complicated singleton pregnancies, and it doesn’t carry the same long-term consequences.
Blood pressure is a good example. The fetal-origins hypothesis predicts that lower birth weight should lead to higher blood pressure in adulthood. But when researchers compared blood pressure in twins to their singleton siblings from the same families, no differences emerged, despite a birth weight gap of more than a kilogram on average. The result held for both sexes and across age groups, and adjusting for adult body size didn’t change it.4PubMed. Comparing blood pressure of twins and their singleton siblings: being a twin does not affect adult blood pressure A separate study of young Swedish male twins found that within twin pairs, birth weight differences had essentially no relationship with systolic blood pressure differences in adulthood, providing little support for the fetal-origins hypothesis in this population.5PubMed. Fetal growth and systolic blood pressure in young adulthood: the Swedish Young Male Twins Study
The cardiovascular picture gets more nuanced when you separate identical from fraternal twins. A large Swedish study found that lower birth weight was linked to higher cardiovascular risk within fraternal pairs but not within identical pairs. In fraternal twins, a one-kilogram increase in birth weight within a pair was associated with roughly 25% lower odds of coronary heart disease and about 40% lower odds of stroke. Among identical pairs, the same analysis showed no association at all.6PubMed. Birth weight predicts risk of cardiovascular disease within dizygotic but not monozygotic twin pairs: a large population-based co-twin-control study This suggests that whatever links birth weight to cardiovascular disease involves factors that differ between fraternal twins (like genetics and placental setup) rather than something about being born small per se.
Diabetes and Metabolic Health Stand Out
If there’s one area where twins might face a genuine disadvantage, it’s metabolic health, particularly type 2 diabetes. A Danish study of elderly twins found they were more insulin resistant and glucose intolerant than matched singletons, and their prevalence of type 2 diabetes was substantially higher: roughly 17% among identical twins and 16% among fraternal twins, compared to about 6% in singletons. Twins also had about 60% higher incidence of new diabetes diagnoses during follow-up.7PubMed Central. Increased risk of type 2 diabetes in elderly twins
This finding wasn’t limited to older adults. A study of young twins and singletons in Guinea-Bissau found that twins had higher fasting and post-meal glucose levels. Impaired fasting glucose was about three and a half times more common in twins, and overall dysglycemia was roughly twice as prevalent.8PubMed Central. Prevalence of impaired glucose tolerance and other types of dysglycaemia among young twins and singletons in Guinea-Bissau
But the picture isn’t entirely straightforward. A study of monozygotic twins with extreme birth weight differences found no meaningful differences in glucose metabolism between the smaller and larger twin of each pair once you accounted for shared genetics and upbringing.9PubMed. Adult glucose metabolism in extremely birthweight-discordant monozygotic twins That seems to contradict the idea that low birth weight itself causes metabolic problems. Meanwhile, a separate study of twins genetically susceptible to type 2 diabetes found that within-pair birth weight differences did predict who went on to develop abnormal glucose tolerance, with a nearly ninefold higher risk of metabolic syndrome in pairs where the birth weight gap exceeded 450 grams.10PubMed. Low birthweight and metabolic abnormalities in twins with increased susceptibility to Type 2 diabetes mellitus
How to reconcile these findings? The diabetes risk in twins may depend heavily on genetic susceptibility. In genetically predisposed individuals, the metabolic stress of a twin pregnancy could tip the scales. In the broader twin population, the effect is inconsistent enough that it doesn’t translate into the kind of across-the-board health penalty you’d expect if being a twin were inherently harmful. Still, the diabetes signal is the most convincing evidence that twinning can leave a metabolic trace into later life, even if the mechanism remains debated.
Mental Health and Cognitive Ability
Parents of twins sometimes worry about developmental delays, and premature birth (which is more common in twins) can raise the risk of early cognitive challenges. But by adulthood, twins appear to perform cognitively at the same level as singletons. A family-based comparison of adult twins with their singleton siblings found no differences in IQ scores, suggesting that any early developmental gap closes over time.11Intelligence. Do Twins Have Lower Cognitive Ability than Singletons?
Psychiatric health follows a similar pattern. A study comparing self-reported psychiatric symptoms in twins and their non-twin relatives found that levels and variability of common symptoms were essentially the same, with the possible exception of slightly elevated scores for panic and phobia in twins.12PubMed. Self-report psychiatric symptoms in twins and their nontwin relatives: are twins different? A population-based study in Sri Lanka found comparable or slightly lower rates of depression, anxiety, and PTSD among twins than singletons.13PubMed Central. The Colombo Twin and Singleton Follow-up Study: a population based twin study of psychiatric disorders and metabolic syndrome in Sri Lanka
In childhood, the evidence is even more encouraging. A large population-based study found that twins may actually have a lower rate of behavioral problems than singletons, at least according to teacher assessments. Twin boys were rated as less disturbed than singleton boys, and twin girls scored lower on emotional disturbance measures.14PubMed. Are twins’ behavioural/emotional problems different from singletons’? The researchers speculated that having a constant companion from birth might provide a buffer against some behavioral difficulties. Whatever the explanation, the idea that twins are psychologically fragile doesn’t hold up in the data.
When Sharing a Placenta Raises the Stakes
Not all twin pregnancies are created equal. About two-thirds of identical twins are monochorionic, meaning they share a single placenta. This arrangement creates risks that dichorionic twins (who each have their own placenta) and fraternal twins simply don’t face.
The most serious complication unique to monochorionic twins is twin-to-twin transfusion syndrome, where blood vessels in the shared placenta create an imbalance: one twin (the “donor”) gets less blood and nutrients, while the other (the “recipient”) gets too much. Even after successful treatment with laser surgery to seal off the abnormal blood vessel connections, long-term consequences can linger. Recipient twins may develop heart problems including obstruction of the right ventricular outflow tract, while both donor and recipient twins face elevated risks of neurodevelopmental impairment.15PubMed. Long-term outcomes for monochorionic twins after laser therapy in twin-to-twin transfusion syndrome
Even without twin-to-twin transfusion syndrome, monochorionic twins carry a higher risk of cerebral palsy. In one study, about 2.2% of monochorionic survivors developed cerebral palsy compared to 0.5% of dichorionic twins. The cases were linked to complications specific to the shared placenta: fetal death of a co-twin leading to preterm delivery and brain hemorrhage, and severe transfusion syndrome.16PubMed Central. Long-Term Neurodevelopmental Outcome of Monochorionic and Matched Dichorionic Twins These are rare outcomes, but they represent a genuine health burden that is specific to a subset of identical twins and has nothing to do with twinning in general.
The distinction matters because when studies report that “twins” have higher rates of certain neurological conditions, the signal is often driven almost entirely by monochorionic pairs affected by placental complications. Fraternal twins and dichorionic identical twins are essentially in the clear on these fronts.
The Twin Protection Effect
Here is where the story takes an unexpected turn. Rather than being disadvantaged, twins may actually live longer than singletons, at least in some populations. An analysis of U.S. World War II veteran twins found that identical twins had a median lifespan of 82 years, compared to 80.5 years for fraternal twins. Frequent communication between identical twin partners, along with better exercise habits and lower smoking rates, helped explain the gap.17The Journals of Gerontology: Series A. Communication Between Identical Twins: Health Behavior and Social Factors Are Associated With Longevity That Is Greater Among Identical Than Fraternal U.S. World War II Veteran Twins
A demographic modeling study found protective effects for twins at multiple stages of life. At younger ages, both male and female twins appeared protected against what the researchers called “extrinsic” causes of death (accidents, violence, and similar external threats). At older ages, identical male twins showed additional protection against intrinsic causes like disease. The researchers compared this to the well-documented marriage protection effect, where married people live longer than unmarried people, likely because of social support, accountability, and shared resources.18PubMed Central. A Twin Protection Effect? Explaining Twin Survival Advantages with a Two-Process Mortality Model
The implication is that having a built-in close companion from birth provides many of the same health benefits as a strong marriage. Someone who knows your habits, notices when something is off, and gives you a reason to take care of yourself. This social dimension of twinning is often overlooked in medical discussions that focus exclusively on prenatal risk factors.
Autoimmune Disease and Bone Health
For conditions outside the big killers, the evidence is reassuring as well. Rheumatoid arthritis, a chronic autoimmune disease, occurs at virtually identical rates in twins and singletons. A Danish twin study found a prevalence of 0.37% in twins, compared to a population estimate of 0.35% in singletons, using the same classification criteria and drawn from the same ethnic background.19PLOS ONE. On the Origin of Rheumatoid Arthritis: The Impact of Environment and Genes—A Population Based Twin Study
Bone health is an area where twin studies have been enormously informative, though more for what they reveal about genetics than about twin-specific risks. Twin studies have established that genetic factors account for a large portion of the variation in bone mineral density across the skeleton, with the spine showing a stronger genetic contribution than the hip or forearm. Environmental factors like smoking, exercise, and body composition explain the rest.20PubMed Central. Genetic determinants of bone mass in adults. A twin study A co-twin study of women found that each extra decade of smoking was associated with a 2 to 3% drop in bone density, while each additional hour per week of sports was linked to nearly a 1% increase at the hip.21PubMed. Determinants of bone density in 30- to 65-year-old women: a co-twin study These studies use twins as a tool for understanding bone health in everyone; they don’t suggest that being a twin is itself a risk factor for osteoporosis.
Hospitalizations and the Preterm Birth Factor
One Israeli study did find that twins had slightly higher hospitalization rates across several disease categories compared to singletons, including cardiac, respiratory, neurological, and infectious conditions. After adjusting for potential confounders, adjusted hazard ratios ranged from about 1.09 to 1.75 depending on the category, with malignancies showing the largest relative increase (though from a very small base rate of about 0.7% vs 0.4%).22PubMed Central. Twins vs singletons—Long‐term health outcomes
The critical detail is that the researchers attributed this increased risk largely to twins’ higher likelihood of being born preterm. Preterm birth is the single biggest medical risk associated with being a twin, and it has well-established downstream effects on respiratory function, neurological development, and susceptibility to infection in childhood. The question of whether twins face health problems later in life, then, may often be a question about prematurity rather than about twinning itself. A twin born at full term with a healthy birth weight is in a fundamentally different position than one born at 28 weeks needing intensive care.
How Identical Twins Diverge Over Time
One of the most striking findings in twin research has nothing to do with twins versus singletons and everything to do with how genetically identical people can end up with different health trajectories. A landmark study found that while young identical twins are essentially indistinguishable in their epigenetic profiles, older identical twins show dramatic differences in DNA methylation and histone modification patterns across their genomes.23PubMed Central. Epigenetic differences arise during the lifetime of monozygotic twins These epigenetic changes affect which genes are turned on or off, altering everything from disease susceptibility to physical appearance.
This epigenetic drift helps explain why one identical twin might develop an autoimmune disorder, a cancer, or Parkinson’s disease while the other stays healthy. A study of Parkinson’s disease in twins found that when at least one twin was diagnosed after age 50, genetic factors played a minor role, with concordance rates similar between identical and fraternal pairs. Only when the disease struck before age 50 did genetics clearly matter, with all four identical pairs concordant compared to about 1 in 6 fraternal pairs.24PubMed. Parkinson disease in twins: an etiologic study A 20-year follow-up of the same cohort confirmed this pattern, with heritability estimates much higher for early-onset disease than for the more common late-onset form.25PubMed. Concordance for Parkinson’s disease in twins: A 20-year update
The broader lesson from the Swedish Twin Registry is consistent: when researchers tried to quantify how much genetics contributed to lifespan overall, the answer was at most about a third, with the rest attributable to individual environmental factors, meaning the experiences unique to each person rather than anything shared by the family.26PubMed. The effect of genetic factors for longevity: a comparison of identical and fraternal twins in the Swedish Twin Registry Your genes set a range. What you do with your life, from diet and exercise to where you live and what you’re exposed to, fills in the specifics. Being a twin doesn’t change that equation.
Fertility in Twins
One area where twins do show a measurable difference from singletons is reproduction, though it only applies to one group. A Finnish population study found that female fraternal twins had modestly higher completed fertility than both singletons and female identical twins, and were more likely to have a twin birth themselves. Male twins showed no meaningful fertility differences. And the elevated fertility in female fraternal twins did not pass to the next generation, meaning it was specific to the twin generation rather than a heritable trait running through families.27medRxiv. Fertility rates across generations in twins and singletons: A total population study in Finland The likely explanation is that fraternal twinning is associated with higher levels of follicle-stimulating hormone, which can independently affect fertility. This is a quirk of the biology that produces fraternal twins rather than a consequence of being raised as a twin.