Those who have not had a threesome may imagine it as a straightforward matter of multiplication: the connection and sensation of sex between two people, multiplied by 150%. Couples especially may regard the third party as auxiliary, an add-on to augment their pleasure. But this glosses over the vast potential for complexity, beyond FFM or MMF gender composition. (The fact that FFF or MMM are barely spoken of speaks for itself.)

Participants may also differ on how much prior discussion is desirable, what is and isn’t out of bounds, and for whom – even on who is to be the center of attention. “Different people can be into threesomes for very different reasons,” says Lehmiller. In fact, the most common sexual fantasy may also be the most misunderstood.

Last year Dr Ryan Scoats, of Coventry University, published Understanding Threesomes – the first in-depth study on threesomes in 30 years. He had found that the motivations went far beyond straightforward novelty-seeking, from ticking it off a sexual “bucket list”, to settling psychological debts – for example, by levelling the score with an unfaithful partner. One woman who had cheated on her husband in a threesome later repeated the experience with him as payback.