Separated at birth
Before the 1960s, twins offered for adoption were often separated at birth, and brought up by different families. Scientists interested in human behaviour are studying these twins. If genes are more important in behaviour, then separated twins should have similar personalities, despite their different families. But if environment is more important, then each twin should be more like their adoptive family. In fact, it is neither genes nor environment alone that shape our personalities, but the complex interactions between them.