Asked by: Nick Wilthew, Newport Pagnell
Waving back is a sort of ‘affiliative behaviour’, something that promotes bonding between members of a group. Experimentally, it’s seen more in pleasant, non-urban environments: such studies implicate mood, although they don’t assess happiness directly. From the other side, several studies have manipulated mood to investigate social interaction, although I couldn’t find any specifically looking at mood and affiliative behaviour. So it’s a great research suggestion! I would expect to see a correlation, if only because unhappy or depressed people often avoid social contact. What’s more, people usually seek ‘mood congruent’ activities: they’re drawn to cheerful things when happy, to sad ones when unhappy. Happiness even causes people to evaluate social interactions in a positive light. But you should bear in mind that experimental subjects also seek ‘mood incongruent’ stimulation before meeting strangers, probably to moderate their mood (‘cool’ is usually more socially acceptable). Also, your basic personality will probably be much more influential: some people may always wave back, others never.
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