used the average number of different people phoned during a month (essentially the sympathy group, or 15-layer) to explore changes in network size with age.

Monica Whitty examined the traits that make victims vulnerable. She found that around two-thirds of victims are women. They tend to be middle-aged, to score high on impulsivity and sensation-seeking, to be less kind but at the same time more trusting, and to have an addictive personality (which is likely to make them more clingy).

1,500-layer is the number of faces you can put names to

Bonded relationships, and the sophisticated cognition that underpins these, are the key solution that primates, in particular, evolved to maintain social groups.

People’s texting patterns looked just like their phone-call patterns: each person had an individual fingerprint in the way they distributed their texts among their friends and family, just as they did with their phone calls.

Without trust, we cannot forge friendships; but if we trust everyone unconditionally, we will in the end fall prey to freeloaders who exploit our trust for their own ends.

A relationship’s resistance to breaches of trust was viewed as a function of the level of trust: the more trusting the relationship, the bigger or more frequent the breaches had to be to destabilise it.

Since all relationships, and especially those with strangers, are potentially risky (we don’t know quite how they are going to behave), the initial instinct will always be to run away.

simultaneously. If we couldn’t, we wouldn’t be able to worry about the consequences of our behaviour for our other friendships. There would be no such thing as guilt – not just because we feel guilty about breaking some abstract law, but because we worry that our actions may hurt or upset someone, or that others will think badly of us.