for participants to answer honestly. Then there was the issue of
sexual terminology. ‘There was that mismatch between the public
health language and the language of everyday, which was so full of
euphemisms,’ Wellings noted. She recalled that several participants
didn’t recognise terms like ‘heterosexual’ or ‘vaginal’. ‘All the Latin-
sounding names, or any word with more than three syllables, was
thought of as something completely weird and unorthodox.’
Yet the Natsal team did have some advantages, such as the
relatively low frequency of sexual encounters. The most recent Natsal
study found that a typical twenty-something in the UK has sex about
five times a month on average, with less than one new sexual partner
per year.[12] Even the most active individuals are unlikely to sleep
with more than a few dozen people in a given year. It means that
most interviewees will know how many partners they’ve had and what
those partnerships involved. Contrast that with the sort of interactions
that might spread flu, such as conversations or handshakes. Each
day, we may have dozens of face-to-face encounters like these.
During the past decade or so, researchers have increasingly tried
to measure social contacts that are relevant for respiratory infections
like flu. The best known is the study, which asked over
7,000 participants in eight European countries who they interacted
with. This included physical contacts, like handshakes, as well as
conversations. Researchers have since run similar studies in
countries ranging from Kenya to Hong Kong. The studies are also
getting more ambitious: I recently worked with collaborators at the
University of Cambridge to run a public science project collecting
social behaviour data from over 50,000 volunteers in the UK.[13]
Thanks to these studies, we now know that certain aspects of
behaviour are fairly consistent around the world. People tend to mix
with people of a similar age, with children having by far the most
contacts.[14] Interactions in schools and at home typically involve
physical contact, and encounters that occur on a daily basis often last
longer than an hour. Even so, the overall number of interactions can
vary a lot between locations. Hong Kong residents typically have
physical contact with around five other people each day; the UK is
similar, but in Italy, the average is ten.[15]