Identical twins share 100 per cent of their genes - but can grow to look less similar as they age, and have different personality traits and susceptibilities to disease. This makes them a unique resource to study the separate genetic and environmental influences that shape us. Trust-funded twin studies are therefore focussing on the differences between identical twins in search of clues to how we become who we are.

Researchers believe that some of the differences in the appearance and behaviour of identical twins are due to changes in the activation of genes rather than the genes themselves. These are known as epigenetic changes because they change how the genome operates without changing its actual sequence. Our epigenomes therefore refer to the set of unique operating instructions that run each of our genomes' 'hard drive' differently.

Since a genome can pick up or shed an epigenetic change much more rapidly than it can change its DNA sequence, this may supply a mechanism by which we can mould our identities through the lifestyle and behavioural choices we make. Our genes may be fixed, but our genetic identities have a degree of fluidity, enabling us to make choices about who and what we become throughout our lives.