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What determines a person's height?

girl getting height measured at doctor's office
(Image credit: FS Productions via Getty Images)

People of diverse heights populate the world, so why is it that some adults are short, while others are as tall as professional basketball players?

Researchers looking for the answer have found it largely relates to genes. Indeed, studies looking for metrics and variables that can be used to predict a person's height have found that genetics is a powerful indicator.

In other words, people with tall parents will likely also need to pay for extra legroom on a flight. But there's another important consideration: This assumes that you didn't experience some sort of serious hardship in your early years. Studies have shown that malnourishment  and severe disease during a person’s childhood can prevent them from reaching their genetic potential for height. 

Related: Does coffee really stunt kids' growth?

These hardships can even affect average changes in height across entire countries. In a 2016 study in the journal eLife, carried out by researchers from Imperial College London, analyses showed that the world's tallest men come from the Netherlands and the world's tallest women hail from Latvia. But this league table wasn't always this way, according to the international Non-Communicable Diseases Risk Factor Collaboration (NCD-RisC). 

South Korea was 133rd in the 1985 rankings, but by 2019, it had jumped to 60th place. The leading theory among scientists, at least, is that this boost may be due to improved diets resulting from South Korea's development in recent decades. 

"In South Korea and the People's Republic of China, it's widely understood that increasing height over the last one to two generations is largely due to improved nutrition," said Stephen Hsu, a professor of computational mathematics, science and engineering at Michigan State University whose research has looked at predicting a person's height. "Protein and calcium and total calorie intake have all gone up a lot during that time." 

Meanwhile, other countries have fallen on the NCD-RisC's list. In 1985, for example, the United States was the 38th-tallest country in the world, but in 2019, it dropped to 58th place. Is that because the country has experienced immigration from other countries where people are smaller on average? And in that case, is it all about genetics? That's likely one main factor, but not the only one, Hsu said.

"Aside from immigration, some people hypothesize that the quality of nutrition for everyone has gone down with the increased consumption of fast food, soft drinks, etc.," Hsu said. It could also just be that other countries are edging out the U.S. In other words, Americans aren't necessarily getting shorter; they’re just not growing as fast as people in other nations. 

Poor nutrition isn't the only environmental factor that can affect a person's stature. Serious  diseases can also negatively affect growth, especially if they occur in childhood; celiac disease, bone disease, such as rickets and juvenile osteoporosis, and anemia are all examples

Although an unhealthy diet and serious illness in childhood can lead to a shorter stature, research suggests that genetic coding is far more influential.

In a 2018 study published in the journal Genetics, Hsu showcased the importance of genes in the determination of height. Along with colleagues, he used machine learning and computer algorithms to analyze close to half a million genomes of people living in the United Kingdom. After crunching the numbers, the team was able to accurately predict a person's height and bone density from just their genes. 

Moreover, genetic mutations and hormonal imbalances have also been linked to short stature, including dwarfism, a condition in which a person stands at 4 feet, 10 inches (147 centimeters) tall or less. Dwarfism can be split into two subtypes. First, there's what is known as disproportionate dwarfism, which is when some parts of the body are small, but others are either of average or above-average size. The other type is proportionate dwarfism, in which all body parts are proportionately smaller than average. Both types of dwarfism are genetic, and there are 200 or so genetic variations that can end up causing the two forms, according to Stanford at The Tech. Some of these genes are genetically dominant, meaning a person needs to inherit the genes from only one parent, whereas others are genetically recessive, meaning a person needs to inherit the genes from both parents. 

On the opposite end of the spectrum, people can grow to be giants. Take Robert Wadlow, the tallest person in recorded history, who stood 8 feet, 11.1 inches tall (272 cm). Excessive growth like this, sometimes called gigantism, can be a sign of cancer. Children with tumors in their pituitary glands, for example, can end up overproducing growth hormones.

Barring medical conditions, for well-nourished people, "It appears that genetics largely determines adult height," Hsu said. 

Originally published on Live Science.