Scientists see twins as the perfect laboratory to examine the impact of nature vs. nurture.

Every August for the past 43 years, Twinsburg, Ohio, has hosted the biggest gathering of twins in the world. Two decades ago, organizers added an attraction to the lineup of parade, talent show and hot-dog dinner that drew more than 2,000 pairs this year: the chance to participate in research. Scientists vie for tent spots to test such things as twins’ exposure to the sun, their stroke risk and their taste preferences. “Every year we get more [research] requests than we can handle,” says Sandy Miller, a Twins Day Festival organizer and mother of 54-year-old twins. “We just don’t have room for all the scientists who want to come.”

Since English scientist Francis Galton published a paper on the heritability of traits in 1875, researchers have been fascinated by how the behavior and health of identical twins differ throughout their lifetimes.

“Twins are nature’s experiment,” says Australian neuropsychiatrist Perminder Sachdev, who runs the Older Australian Twins Study, which was started 10 years ago and has recruited more that 300 pairs of twins older than 65 to analyze how physical activity, psychological trauma, alcohol use and nutrition affects their brains, psyches, metabolisms and hearts.

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