One of the biggest puzzles in science is distinguishing what is caused by nature versus nurture. That's why a lab here with a treasure trove of data on twins is increasingly fielding requests from around the world from researchers trying, for example, to uncover causes of dementia and better understand economic behaviors like financial risk-taking.
The Swedish Twin Registry, launched 50 years ago, contains birth data, medical records and other information on nearly 100,000 pairs of twins—believed to be the largest and oldest such resource in the world. The data have spawned over 500 published reports examining various questions about cancer, asthma, family relationships and mental health. Early findings provided valuable evidence linking cigarette smoking with lung cancer, and helped to dispel the so-called refrigerator-mother theory, a popular belief in the 1950s that emotionally frigid mothers were to blame for their children's autism.
Identical twins share all the same genes. So, if a disease is fully genetic-based, both twins should have the same risk of contracting it. More commonly, however, genes and environmental factors both play a role. Recent research found that the identical sibling of a twin with dementia has about a 50% chance of also developing this type of cognitive impairment, indicating that genetics are only part of the cause of the disease. By contrast, fraternal twins and other nonidentical siblings, who share roughly half of the same genes, have about a 25% chance of getting dementia if the other sibling has it.
Studies of twins allow researchers to explore "the eternal question of nature or nurture—why we are the way we are," says Paul Lichtenstein, the director of the registry. The registry, housed in the Department of Medical Epidemiology and Biostatistics at the Karolinska Institute here, receives applications seeking access to the database from scientists world-wide, including those working in the currently hot areas of gene sequencing, or mapping of a person's full genetic code, and epigenetics, which studies how genes can be altered by environmental factors. Other twin registries are growing around the world, including relatively new ones in several countries in Asia.
More on Medicine's Genetic Revolution
Besides basic health information, the registry has collected about 45,000 DNA samples of twins in its biobank freezers. Currently the samples are used to hunt for specific genetic variants between twins that might account for health-related behaviors and conditions, such as how much people eat or how much coffee they drink, says Patrik Magnusson, the registry's head of research. As sequencing technology advances and becomes more affordable, researchers expect someday to readily decode twins' full genomes to further identify genetic factors of diseases.
The registry also draws researchers from outside the medical field. David Cesarini, an economist at the Center for Experimental Social Science at New York University, Magnus Johannesson of the Stockholm School of Economics and others have used the database to investigate whether people's genes play a role in determining fairness and financial risk-taking. So far, "the studies establish that genetic factors taken as a whole are at least moderately correlated with things like attitudes towards risk or altruism," says Dr. Cesarini.
The Swedish Twin Registry costs about $2 million a year to run, paid for by the Swedish government and grants for specific projects from international science foundations. Drug companies also have provided funds as part of efforts to investigate biological markers of disease.
The registry was founded in the late 1950s, when two Swedish researchers, with funding from the country's tobacco industry, wanted to study the then-inconclusive evidence that smoking was bad for health. The goal was to sort out whether some people were genetically more likely to get cancer or whether there were potential health effects induced by chemicals in tobacco smoke. If twins had similar rates of cancer, despite differences in smoking habits, it would have suggested that smoking wasn't carcinogenic.
The researchers had a vast database to work with. As early as the 1800s, twin births in Sweden were recorded by priests in town parishes. That record-keeping continues in modern times in Sweden and several other European countries, along with recording basic health information of residents. The researchers tracked down about 11,000 pairs of twins born between the years of 1886 and 1925 and asked about their smoking behaviors and health. Eventually the work showed that smoking did increase the risk of cancer, and funders for the project pulled out, Dr. Lichtenstein says.
Renewed interest in the registry came in the early 1980s with the arrival of Nancy Pedersen, a psychological researcher from the U.S. who began studying differences between identical twins who were adopted and reared separately. Dr. Pedersen, a professor in the department of medical epidemiology and biostatistics at Karolinska Institute, currently studies dementia among twins, including the recent research showing that an identical sibling of a twin with dementia has an increased risk of also developing the condition.
Some of the early findings about twins reared separately were surprising, Dr. Lichtenstein says. In one study, he and colleagues found that genetics played a significant role in whether a person experiences difficult life events, like the death of a spouse, unemployment and being a victim of crime.
People's genetic makeup affects characteristics like intelligence, talent and how sociable they are. That, in turn, affects their socio-economic status and life circumstances, he says. Dr. Lichtenstein, a sociologist, says he previously believed that environmental factors, like social class and education, were far more important than genetic makeup. The findings don't mean that victims of adverse experiences cause those events, but that the situations aren't based only on luck or unfortunate choices. "We are shaping our own environment in a way," says Dr. Lichtenstein.
Researchers are currently studying asthma in twins, who typically develop at slightly different rates. A recent study showed that the bigger twin at birth is less likely to develop asthma by the age of 9 years than the smaller twin. But the bigger twin is more likely to develop eczema, an autoimmune disease, according to Catarina Almqvist, a professor and pediatrician who led the study. Dr. Almqvist suggests that the smaller twin has lungs that are less fully developed and thus has a greater chance of developing asthma. Babies with restricted fetal growth have a more immature immune system and won't develop eczema as easily as the twins without impaired fetal growth.
Dr. Almqvist is continuing her work with this group of twins to test the so-called hygiene hypothesis, which suggests that people's immune systems suffer in an overly clean modern environment. She is studying fecal samples taken from the twins to see if differences in quantity and variety of bacteria are associated with asthma and autoimmune disease.
Corrections & Amplifications
Nancy Pedersen, a professor in the department of medical epidemiology and biostatistics at Karolinska Institute, conducted a study at the Swedish Twin Registry showing that an identical sibling of a twin with dementia has an increased risk of also developing the condition. An earlier version of this column incorrectly said that Paul Lichtenstein, the registry's director, performed that research.
Write to Shirley S. Wang at email@example.com