For years, the Netherlands has held the world title for having the tallest people on the planet. But new data from the office for national statistics suggests that the height of the average Dutch person is shrinking. And scientists are puzzled as to why.

At just over 6 feet for men and about 5-foot-6 for women, the Dutch are still the world’s tallest population. But the growth that has seen the country to the top of global height charts for decades appears to have ground to a halt.

“In the course of the last century we have become taller and taller, but since 1980 the growth has stopped,” government statisticians said Friday in a report on the findings written in Dutch.

Dutch women born in 2001 are, on average, more than half an inch shorter than those born in 1980, while for men the decline is 0.39 of an inch, or 1 centimeter.

The statisticians said the decrease relates partly to “the increased immigration of shorter new population groups and the children born from them in the Netherlands.”

Still, that doesn’t account for why growth also stagnated in the generations in which both parents were born in the Netherlands or in the generations in which all four grandparents were born in the Netherlands. “Men without a migration background did not get any taller, and women without a migration background show a downward trend,” the statisticians added.

The new data were based on a number of surveys by health officials of 719,000 Dutch-born individuals between the ages of 19 and 60, who self-reported their height, and used the average height at age 19 as a benchmark.

Scientists have offered a number of explanations — including the possible economic ramifications of the 2007 financial crisis, the increased consumption of unhealthy food or even a shift from meat to plant-based diets, while noting that such theories are speculative at this stage.

Previous studies have shown that while the rest of the world got taller in recent years, Americans plateaued — growing heavier rather than taller; a change some experts have pegged to poor nutrition and shifts in migration. The average American man between the ages of 20 and 39 weighed about 197 pounds and stood at 5 feet 9 inches tall, according to a 2015-2016 health survey by the National Center for Health Statistics. The average woman of that age range was roughly 5 feet 4 inches tall and weighed about 168 pounds.

Such height studies are important, scientists say, because taller people generally live longer. They are less likely to have difficult pregnancies or to develop heart or respiratory diseases. Taller people may also earn more money and be more successful in school.

Majid Ezzati, an expert in global environmental health at Imperial College in London, said it would take a few more years of data to confirm whether the Dutch are experiencing a new trend. If they are, he said, it will most likely come down to nutrition. He told the Guardian newspaper that a Dutch school milk program was thought to be one reason the population had grown so tall in recent decades. In recent years, though, demand for fast-food has boomed.

It remains unclear whether poorer nutrition is limited to certain demographics, Ezzati said, or whether it reflects new fashions and social trends nationwide. He downplayed the role of migration, saying it wasn’t of a scale that would lead to a change in height.

Gert Stulp, an expert at the University of Groningen’s faculty of behavioral and social sciences, said that similar height trends in the United States indicate that fast food could be a factor.

“Diets may have changed,” Stulp told the Guardian. “This is believed to be the reason why the Americans are shrinking; poorer diets, more calories, but fewer nutrients.”

Previous studies have suggested that plant-based diets also could be playing a role, although Stulp said that remains speculative and “there is no evidence for that.”

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