In the battle to find what causes mental illness, scientists are increasingly looking at genetic factors.
For James Longman - whose father killed himself after suffering from schizophrenia - it's a very personal question.
I'm often told I look like my dad, that I have his mannerisms and some of his habits. It's something I take pride in. But it's also something that worries me because he had schizophrenia, and when I was nine, he took his own life.
After a particularly bad two-week episode, he set fire to his flat in London, and threw himself out of a window.
Some of the details of his life and death have only become clear while looking into this story. Multiple suicide attempts; walking around London in just a bathrobe; hearing voices. They are details that contrast so strongly with the man I remember from when he was well - happy, creative and funny.
Twenty years earlier, his own father - my grandfather - had shot himself after finding out he had cancer. I also have other family with mental health issues. Now in my twenties I sometimes struggle with depression. So I naturally think - is this something that runs in my family?
For a lot of people, mental health is a difficult thing to talk about. But those who deal with these issues can often point to family members with similar problems.
Do I get depressed because of the trauma of losing my father in such tragic circumstances? Or is it written into my DNA?
At King's College London (KCL), researchers have been looking into the genetics of mental health.
Studies into twins and family histories have proved that mental health illnesses have a genetic contribution. But it's only in the past few years scientists have been able to fully identify the genetic changes that might be causing that increase in risk.
Prof Cathryn Lewis, a researcher from the NIHR Maudsley Biomedical Research Centre explains: "It's really hard to identify the genetics for mental health disorders. We learn at school about simple Mendelian [relating to the laws of Gregor Mendel] diseases - like Huntington's or cystic fibrosis - where there is the gene, a single gene that contributes to it.
"Mental health disorders are not about a single gene but about a collection of genes. We need to start thinking about this as a cumulative loading of genetics."
The work at KCL is still in its early stages. But it has been found there are 108 genes with changes in people with schizophrenia. Now, nine genes have been found across those with depression, and 20 in people who have bipolar disorder. There are almost certainly many more still to find and scientists say many of these genes will be shared across the different conditions.
Making sense of my genetic legacy is important, because it allows me to break away from the feelings of inevitability that depression gives you. Is this meant to happen? Am I meant to get over this? Is this bigger than me?
"Evidence from the last few years also suggests that many mental disorders share common genetic risk factors - for instance, genetic variation associated with schizophrenia overlaps with both depression and bipolar disorder," says Lewis.
My father's schizophrenia has not been passed down to me - as I haven't experienced any of the symptoms I would have had by now - but perhaps some of the genetic coding we share has caused me to experience depression.
Siblings can have very different outcomes. Twins Lucy and Jonny have a bipolar mother. Jonny has also suffered from the condition, while Lucy has not.
"When I have a bad episode I'm not able to drag myself out of bed," says Jonny. "Or if I do, maybe next level, I'm able to drag myself out of bed but I can't understand things. I literally can't understand how things fit together. There's cloudiness in my brain. So there are lots of physical aspects of feeling what people are calling mentally unwell."
I asked him how he felt when he was diagnosed with his mother's illness.
"Oh my God. So many different emotions and intellectual responses that you go through. I cried with relief." But he continued: "I'm my mother's child. But I feel like my own condition is unique. Because every mental health condition is unique to that person."
If you have a depressed parent, you're twice as likely to experience depression. With bipolar, you're four times more at risk. And in schizophrenia - which my dad had - it's eight times.
But these are relative risks - overall risk remains very low. And, as ever, there's a certain amount of chance involved in picking up genetic traits. Our upbringings and what we experience as children and adults has a huge influence on whether or not we develop mental illness.
My father's death has had a huge impact on my life and on my relationship with my mother. She too has depression - brought on perhaps by my father's death. So it was vital that I spoke to her, to understand a little more of what she went through - and what worries she had about me.
The sense of failure she felt in not being able to stop his death was clear. So too were her concerns about me developing a psychotic illness like my dad.
She describes the day he died. She came into my school to tell me, and remembers my nine-year-old face looking up at her from her lap, my legs swinging between hers. I'd cry at night, she told me, calling out for my dad. I remember none of this.
But her fears about me inheriting a serious condition like schizophrenia are behind her.
"He would be very proud of you now," she says. "Very, very proud you're doing all the things he would never be able to achieve. In his heart, he would feel: 'Oh wow, what a great young man - we made him.'"
