Gert Stulp stands 6 feet, 7 inches tall. His height makes him especially self-conscious at scientific conferences when he rises to describe his research as a demographer at the London School of Tropical Medicine. “It’s always quite embarrassing,” he said.

Dr. Stulp, who is Dutch, studies why his fellow citizens are so tall.

Today, the Dutch are on average the tallest people on the planet. Just 150 years ago, they were relatively short. In 1860, the average Dutch soldier in the Netherlands was just 5 feet 5 inches. American men were 2.7 inches taller.

Since 1860, average heights have increased in many parts of the world, but no people have shot up like the Dutch. The average Dutchman now stands over six feet tall. And while the growth spurt in the United States has stopped in recent years, the Dutch continue to get taller.

For years, scientists have sought to understand why average height has increased, and why the Dutch in particular have grown so quickly. Among other factors, the Dutch have a better diet than in the past, and they also have better medical care. But now Dr. Stulp and his colleagues have found evidence suggesting that evolution itself is also helping to make them taller.