Washington: Want a tall child? Be sure that your spouse is born far away from your town, scientists say.

Polish researchers found that if couples were born in the same town, their children are slightly shorter on average than the kids of parents with far-flung origins.

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 analysed stages of the development period, ranging from 6 to 18 years of age," Danel was quoted as saying by LiveScience.

According to the researchers, 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, who reported their findings in the American Journal of Physical Anthropology, used a dataset of the heights of 2,675 boys and 2,603 girls in Poland, measured yearly in the children's schools. They also collected information on their parental height and family income.

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 per cent of the variations in height among boys and 14 per cent of the variations in height among girls, he said. Marital distance may matter because even in our nomadic, globalised 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 strength 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 kid's genomes for heterozygosity, so marital distance is only an indirect approximation of genetics, Danel said. The researchers are now planning to test
the genetic link directly.