Human history is a tale of upward trajectories: more people, more wealth, more inventions, longer life spans.

But humans seem to have plateaued in one area and may even be going down. According to a new study from Imperial College in London, the average human is getting shorter.

The report looked at 1,472 studies from more than 200 countries that included the measured heights of 18.6 million people between 1896 and 1996. Overall, average human height seems to have peaked about 30 to 40 years ago.

The tallest humans in the report are Dutchmen born before 2000. The shortest were Guatemalan women born before 1900. Iranian men and South Korean women saw the biggest height gains in this period.

Ultimately, the relative decline in height isn’t substantial or definitive. Humans could, conceivably, keep growing if the conditions are right.

So what’s behind this decline? There are several factors at play.

First, human height is heavily influenced by environmental factors such as nourishment in the womb and early infancy. So the gradual climb of global height correlates with gains in nutrition seen globally.

Agricultural breakthroughs in the 20th century, for example, led to tremendous strides in ending hunger.

Read more: What you need to know about Food and Hunger

Still, 795 million people do not have access to enough food to survive and thrive in the world. And 66 million of these people are children, and hunger prevents them from achieving their full potential in school.

That’s one of nine of the world’s population. Meanwhile, humanity produces more than enough to feed everyone.

The reason for shortness in Guatemala is largely due to malnutrition.

Read more: Food waste and the hungry

Next, there’s the genetic variable. If more tall people are having children, then more tall children will emerge. That’s apparently what happened in the Netherlands, where the average man is six feet.  

Elsewhere in the world, this pattern could continue. But some patterns of the past are starting to falter.

People who live in wealthier countries and therefore have better access to nutrition, have fewer children. This could mean that the benefits of living in prosperity — better nutrition — might not be passed onto future generations.

For example, the fertility rate in India, where stunting from malnutrition is rampant, is far higher than it is in Japan, where stunting from malnutrition is exceedingly rare.

However, as food security continues to improve globally, this average height could climb.

By 2100, the human population is expected to grow to 12.3 billion and at that point, gene editing could be making humans much, much taller.