Genes we are born with account for more than half the differences in intelligence between people, a study has shown.
The new findings will fuel the "nature versus nurture" debate over what makes us clever or dumb.
As well as genes, environmental factors such as parenting, nutrition and exposure to chemicals in the womb are also thought to have a significant effect.
The new study highlights the contribution of rare genetic variants which appear to have a disproportionate impact on intelligence.
Scientists examined thousands of genetic markers in the DNA of 20,000 people looking for signals associated with IQ.
They found that the combined effect of rare and common genetic variants explained at least half of the difference in intelligence between individuals.
Dr David Hill, from the University of Edinburgh's Centre for Cognitive Ageing and Cognitive Epidemiology, said: "We used two methods to measure the effect that rare variants had on intelligence.
"By combining the effect of both rare and common variants, more than 50% of the differences in intelligence between people could be traced to their genes."
The study, published in the journal Molecular Psychiatry, is the first to demonstrate the influence rare genetic variants have on intelligence.