Debbie and Trudi are identical twins.
They have much in common, except that Trudi is cheerful and optimistic while Debbie is prone to bouts of profound depression.
It is likely that her depression was triggered by a major life event, though the twins have different views as to what that event might have been.
By studying a group of identical twins like Debbie and Trudi, Prof Tim Spector, based at St Thomas’ hospital in London, has been trying to answer fundamental questions about how our personality is formed. Why are some people more positive about life than others?
Read the whole story: BBC