For decades, ethicists have wrestled with the question of whether society should allow so-called “designer babies”, selected by their parents for desirable genetics. It's important work, because we may eventually face the choice, but a new study suggests the question is not as imminent as often presented.

Some day, parents with the finances may be able to simply buy the version of each gene they want, carefully constructing a baby to suit their ideal, but our capacity for genetic engineering is still far short of that.

A more immediate consideration involves IVF production of several embryos, which parents analyze before implanting the one with the most desirable genetics. Already parents with genetic conditions they don't wish to pass on to their children screen for these, but Dr Shai Carmi of the Hebrew University is interested in the prospects for selecting for positive traits, rather than eliminating negative ones.

"The ability to do genomic sequencing of embryos is much easier than it was even five years ago, and we know many more gene variants linked to certain traits," Shai said in a statement. "But selecting embryos for particular traits is very controversial except when it relates to a serious disease like cystic fibrosis. It raises many issues related to eugenics and unequal opportunities."

The trait most commonly considered for such enhancements is IQ. However, despite the discredited claims of “race realists”, we know very little about the genetic variations that contribute to this, probably because there are so many, each with a minor effect. Carmi ran a simulation by picking real couples as theoretical parents, with each passing a random half of their DNA to 10 embryos.

Carmi found picking the potentially smartest embryo would raise the IQ of the child by 2.5 points – not trivial, but not something most would consider worth the enormous cost.

Height, particularly in men, is also a commonly suggested target for embryo screening. Genetics is thought to be a bigger contributor here, and we know more about which genes are relevant. Nevertheless, Carmi reports in Cell that embryo selection would only account for a 2.5-centimeter (1-inch) difference – significant, but not enough to breed a race of giants.

Moreover, these are best-case scenarios, not assured successes. "There is much about these traits that is unpredictable," he said. Among 28 large families Carmi studied, the child with the greatest genetic propensity for height was seldom the tallest sibling.

Although a smarter population might seem a clear win, opponents of this sort of screening argue that giving an intellectual advantage to the children of those rich enough to afford these costs will do more harm than good by entrenching inequality. Meanwhile, making some people taller provides no obvious social benefit.