When geneticists use the word prediction, they give it a different meaning than the rest of us do. We usually think of predictions as accurate forecasts for particular situations. At a carnival, you might encounter a man who promises to predict your weight simply by looking you over. If you weighed, say, 130 pounds, and he guessed 132, you might be impressed. If he guessed 232, you’d expect to walk away with a giant teddy bear.
Geneticists are a lot more forgiving about predictions. When they try to predict a trait from a set of genes, their prediction may be dead on, or it may be no better than random. Or, as is almost always the case, it is somewhere in between.
Genes that predict the variation in a trait perfectly in a group of people have a predictive power of 100 percent. If they’re no better than what you’d get from blind guesses, their power is zero. The Amsterdam team tested their 52 genes on thousands of people and concluded that the genes have a predictive power of nearly 5 percent. Their predictions are less than random, but if you used them to predict individual people’s intelligence, you’d give away a whole lot of teddy bears.
