"Since the beginning of my pontificate I have had occasion to express my wish to continue to establish bridges of friendship with believers of all religions, showing particularly my appreciation in the belief in dialogue between Muslims and Christians, " he said.

BBC: NEWS | Europe | Pope stresses respect for Muslims

Obviously, there is an extraordinary special relationship between the United States and the United Kingdom, and I am very fortunate that over the last year or two, David and I have been able to, I think, establish an excellent friendship as well.

WHITEHOUSE: President Obama's Bilateral Meeting with Prime Minister Cameron of the United Kingdom | The White House

However, to establish a neutral basis for comparison, we need to take friendship out of the equation for now and consider what would make a basis for fairness between strangers.

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To study friendship in the lab, many use a protocol called "Fast Friends, " which helps strangers establish "interpersonal closeness" in 45 minutes.

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