Who am I?

Twins are born when two babies grow in their mother's womb at the same time. During fertilisation, a single sperm usually fertilises a single egg, which then starts to grow into an embryo, by dividing into two cells, then four, and so on. Occasionally, one embryo splits into two that then grow into genetically identical individuals – clones. Non-identical twins develop if two eggs are fertilised at the same time, by two separate sperm. They have no more genetic similarity than brothers and sisters.

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Scientists can study twins to see how our genes and environment affect our appearance, health and behaviour. They have compared the height and weight of identical twins with those of non-identical twins, for example. Both depend on diet and genes, but to different extents. Your genes do affect your adult weight, but the amount you eat is more important. Your adult height depends on how tall your parents are, although it is also affected by your diet.

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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.

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