On the face of it, David Cameron and Ed Miliband have little in common: one an Old Etonian, Home Counties Conservative, the other a state-school son of north London Marxists.
Yet the Prime Minister has revealed he shares a life-shaping experience with the Labour leader: breaking out of the shadow of his older brother.
In an article for next month's edition of The Big Issue, which Mr Cameron is guest-editing, he also discloses advice his father gave him that could provide useful in the phone-hacking scandal: to remain optimistic "no matter how bad things are".
Mr Cameron wrote The Big Issue's regular feature, My Younger Self, in which contributors offer advice to their 16-year-old selves. Mr Miliband's defeat of his older brother David – long-seen as the future prime minister in the family – in last year's leadership contest is well documented. But similar fraternal tensions in the Cameron family are less well-known.
Talking about his older brother Alex, now a high-flying QC, Mr Cameron writes: "I lived in the shadow of my older brother. He was three years older, and was a huge success on the sports field and almost always lead actor in the school plays.
"It was great to have that kind of role model, and I was incredibly proud of him, but like many younger brothers you find yourself always a few steps behind. If I could give my younger self some advice, I'd say: don't worry about it; your life is not predetermined; you'll find your own feet in your own way.
"It was not until I left school that I felt I was breaking out of my brother's shadow and doing my own thing."
The Prime Minister tells his 16-year-old self: "better to try and fail than not to try at all", recounting how he "didn't always put the effort in" at some subjects "but instead just went through the motions, drifting along, rather than giving it everything".
Mr Cameron presumably got his act together by the time he did his A levels, however, achieving three As and a place at Oxford.
The Prime Minister says he took his family for granted, rather than appreciating how lucky he was to have a stable background.
He writes: "A lot has been written about my background, but the great privilege of my upbringing wasn't just the wealth, it was the warmth. We all got on, we were all there for each other, there was so much love and support.
"I am not sure we all appreciated it enough at the time. I know I get criticised for talking about how important families are to society, but I'm just saying it as I see it, as I experienced it. When you've got a strong family behind you, it makes it easier to cope with the things life throws at you."
The experience of having a disabled father made him focus on the "big picture" and on remaining optimistic. He says his father, Ian Cameron, who died last year, always told him: "no matter how bad things are, you can overcome them if you have the right frame of mind. It was the perfect advice for a future politician.
"In a typical morning, you can wake up to being criticised on the radio, read bad headlines over breakfast and then get skewered in the House of Commons. But throughout it all, you've got to focus on the big picture, do the right thing and remain optimistic."
Turning to his political ideology, he cites a visit to the Soviet Union at the age of 18 that gave him "a sense of what was right and wrong". He adds: "In particular, the importance of freedom and the state being there to serve people, not be their master."
For the edition of The Big Issue, out tomorrow, the Prime Minister has secured contributions from Bill Gates and Michelle Obama.