Want a Tall Kid? Try a Long-Distance Relationship

Check your atlas: The height of your child may be determined, in part, by how far apart mom and dad were born.

If mom and dad were born in the same town, their children are slightly shorter on average than the kids of parents with far-flung origins, a new study finds.

The researchers think the reason boils down to genetics: Parents originating in very different regions likely have very different genes relative to a mother and father who both grew up in the same hometown, where their own parents grew up. That greater genetic diversity may lead to children with bodies that operate more efficiently than others. Energy "saved" by this efficiency could then go to growth, said study author Dariusz Danel, of the Institute of Anthropology at the Polish Academy of Sciences.

"This effect is visible in children during the three analyzed stages of the development period, ranging from 6 to 18 years of age," Danel told LiveScience. [5 Myths About the Male Body]

Danel and his colleagues published their results online in July in the American Journal of Physical Anthropology.

Growing tall
Height is determined by a number of factors, including parents' height and socioeconomic status, because wealthier people tend to be better nourished. Earlier studies had found conflicting evidence about whether marital distance — the distance between parents' birthplaces — matters in how tall children get.

Danel and his colleagues used a dataset of the heights of 2,675 boys and 2,603 girls in Poland, measured yearly in the children's schools. The researchers also had information on parental height and family income. [Read: Taller People Earn More Money]

The latter two factors played an expected role in a child's height, but the researchers also found "unambiguous" evidence that marital distance mattered, too, Danel said. Marital distance explained about 20 percent of the variations in height among boys and 14 percent of the variations in height among girls, he said.

What little boys are made of
Marital distance may matter because even in our nomadic, globalized world, people who are born close to one another tend to have slightly more genetic similarities. Two people born farther from each other would thus be more genetically different, which could confer some genetic strengths on their offspring. This genetic diversity within an individual is called heterozygosity.

Heterozygosity is more likely to express itself in greater height in boys than in girls, Danel said. Boys grow for a longer period of time and at a faster rate than girls do, so any extra energy savings boys get from their diversified genomes is freed up for the energy-intensive growing process. Girls, on the other hand, might shunt their extra energy into reproductive maturation, Danel said, though the current study did not test for this.

Nowadays, most Western kids get more than enough energy through their diets to fuel their growth. But that wasn't always the case, Danel said. For much of human evolutionary history, nutrients were scarce and energy was at a premium. Under those conditions, the genetic strengths conferred by marital distance may have made an even larger difference in height.

The study didn't test children's genomes for heterozygosity, so marital distance is only an indirect approximation of genetics, Danel said. He and his colleagues are now designing an experiment to test the genetic link directly