Scientists said Wednesday they had identified 74 genes that partially determine how far someone gets in school, depending on which variant of those genes a person possesses.

Compared to environmental factors such as diet, family circumstances and opportunity, this hard-wiring has only a meagre influence, accounting for less than half of one percent of the outcome.

Even when combined with all known genetic variants across the human genome, that share only rises to about three percent.

But the findings, published in Nature, are robust enough to help researchers match genetically-linked personality traits -- such as grit and contentiousness -- with education attainment, at least at the level of society, if not the individual.

Even a single gene, they found, could have a measurable impact.

"For the variant with the largest effect, the difference between people with zero copies and those who have two copies predicts, on average, about nine more weeks of schooling," said Daniel Benjamin, a professor at University of Southern California and corresponding author for the consortium that completed the study.

The most common type of genetic variants -- known as SNPs (or "snips") -- can show up as deletions or duplications of DNA fragments.

Earlier research by the same team of 250 scientists worldwide canvassed the genomes of 100,000 people, and only turned up three relevant genes.

This time the Social Science Genetic Association Consortium tripled the number of genomes sequenced, resulting in a much richer haul.

All 300,000 people were of European descent, partly because most data available comes from the United States, Europe and Australia.

There's still a long way to go, explained Peter Visscher, a scientist at the Centre for Neurogenetics and Statistical Genomics at the Queensland Brain Institute in Australia.


Tip of the iceberg

"We know from past studies on twins and other relatives that the total contribution from all genetic variants on individual differences on educational attainment is 20 to 40 percent," he told AFP.

Ramping up sample sizes to millions, rather than thousands, could fill most of that gap, he said by email.

"We have only identified the 'tip of the iceberg' in our study," added Philipp Koellinger of Vrije Universiteit Amsterdam, a co-founder of the consortium.

The genes singled out by the study were strongly active in the brain -- especially prenatally -- and probably play a role in neural development, the researchers found.

Some of these genes also correlated with the risk for dementia, opening up potential pathways for study or treatment, Visscher said.

Other genes predicted risk for bipolar disorder and certain forms of schizophrenia.

The scientists involved in the study anticipated concerns about the potential for discrimination on the basis of genetic profiles, and discouraged drawing straight lines between genes and educational achievement, much less intelligence.

But they said their research could help experts understand how changes in environment magnify -- or reduce -- genetic influences on behaviour, with possible applications for medicine and the classroom.