Understanding friendships, your own included

Personal networks are strongly homogeneous in age. Relationships crossing that barrier have particular properties and are little studied.

The share of single-person households has doubled in half a century in rich countries. Its association with measured isolation is weaker than assumed.

Retirement removes at a stroke the main context for encounters in adult life. Surveys show very different trajectories by situation.

Both events reduce the number of active relationships, but not in the same way nor for the same people.

Surveys find consistent gaps in network composition and in the nature of exchanges. Their interpretation is more disputed than their existence.

The fall in the number of relationships after fifty is consistent across all data. One theory holds that it is partly chosen.

School sociometry produces precise maps of relationships between pupils. They contradict several received ideas about popularity.

Mobile phone data made it possible to trace a curve that self-report surveys suggested without demonstrating. Its shape is more regular than expected.

The definition changes radically between four and twelve. That shift makes comparisons of surveys across ages trickier than it seems.