A new study has found that it may be possible to train people to be more intelligent, increasing the brainpower they had at birth. Until now, it had been widely assumed that the kind of mental ability that allows us to solve new problems without having any relevant previous experience — what psychologists call fluid intelligence — is innate and cannot be taught (though people can raise their grades on tests of it by practicing). But in the new study, researchers describe a method for improving this skill, along with experiments to prove it works. The key, researchers found, was...