In a Swedish lab, Alexander Kotrschal has deliberately moulded the intelligence of small fish called guppies. From a starting population, he picked individuals with either unusually large or small brains for their bodies, and bred them together. It’s what farmers and pet-owners have done for centuries, selectively breeding animals with specific traits, from shorter legs or more muscle.

Or bigger and smaller brains. After just two generations, Kotrschal had one lineage of guppies with brains that were 9 percent bigger than the other. And these individuals proved to be smarter—they outclassed their peers at a simple learning task, where they learned to discriminate between two and four symbols. This may seem like child’s play for us, but it’s a