Many of us categorise ourselves as either optimist or pessimist, but what can science tell us about how we got that way and can we change, asks Michael Mosley.
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?
Spector has been able to identify a handful of genes which are switched on in one twin and not the other.
Twin studies suggest that, when it comes to personality, about half the differences between us are because of genetic factors. But Spector points out that throughout our lives, in response to environmental factors, our genes are constantly being dialled up and down as with a dimmer switch, a process known as epigenetics.
With twins like Trudi and Debbie they have found changes in just five genes in the brain's hippocampus which they believe have triggered depression in Debbie.
Spector, who describes himself as an optimist, hopes that this research will lead to improved treatments for depression and anxiety.
"We used to say," he told me, "that we can't change our genes. We now know there are these mini mechanisms that can switch them on and off. We're regaining control, if you like, of our genes."
Even more surprising is research which has identified changes in the activity of genes caused by the presence or absence of maternal love.
Prof Michael Meaney, from McGill University in Canada, is investigating ways to measure how many glucocorticoid receptors are activated in someone's brain.
The number of active glucocorticoid receptors is an indicator of that person's ability to withstand stress. It may also be a measure of how well mothered they were at a young age - reflecting how anxious and stressed their mothers were, and how this impacted on the amount of affection they received in their early years.
I am one of a small handful of people who have done their test and had the results. I haven't told my mother yet.
I see myself as being more at the pessimistic end of the spectrum but would like to change, so I went to visit psychologist and neuroscientist Prof Elaine Fox at her laboratory at Essex University.
Fox is interested in how our "affective mindset", the way we view the world, shapes us. As well as using questionnaires she and her team look for specific patterns of brain activity.
They began by measuring the levels of electrical activity on the two sides of my brain with an electroencephalograph. It turns out I have more electrical activity in my right frontal cortex than my left. This, Fox explains, is associated with people who are prone to higher levels of pessimism and anxiety.
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