Box 1.

Self-Report and Face-to-Face Measures From Natsal-3 Used in Analysis

CASI (Self-Report) Measures	Response Options (and Recoding)
CASI: Lifetime time frame
“Altogether, in your life so far, how many (women/men) have you had sexual intercourse with (vaginal, oral, or anal)?” Note: Question asked of all women and men regardless of sexual orientation. Analysis includes individuals reporting 0 partners.	Number keyed in. Note: Values capped at the 99th percentile (110 for men, 50 for women) in the analysis, i.e., those in excess replaced by the 99th percentile.
Participants reporting five or more lifetime partners: “Which of these best describes how you worked out that answer?”	Response options: 1. I just knew the number. 2. I remembered each partner and counted them up. 3. I estimated or guessed the number. 4. I remembered some partners and then added on an estimated number for others. Options 1 and 2 are categorized as counting strategies. Options 3 and 4 are categorized as estimating strategies. Note: Analysis showed that, among both men and women, participants who said that they had “remembered some” reported a similar mean number of partners as those who said that they had “estimated or guessed.” In analysis, participants reporting less than five lifetime partners were assumed to have counted, not estimated.
“In your lifetime, how many different (men/women [opposite sex]) have you paid money for sex?”	Number keyed in.
CASI: Last five years time frame
“Altogether, in the last five years, how many (women/men) have you had sexual intercourse with?”	Number keyed in.
“In the last 5 years, have you had sex for the first time, here in the U.K., with anyone who normally lives outside the U.K.?” Include anyone who was visiting the U.K. or living here for a while. If yes: “In the last five years, how many people who normally live outside the U.K. did you have sex with for the first time in the U.K.?”	Yes/no. Number keyed in.
“In the last five years, have you had sex with anyone for the first time while you were in any country outside the U.K.?” If yes: “In the last five years, how many people did you have sex with for the first time while you were in any country outside the UK?”	Yes/no. Number keyed in. Note on analysis: New partners abroad who were U.K. residents were excluded (37%). Among participants reporting new sexual partners from multiple countries while abroad, it was not possible to identify which were U.K. residents, and all these partners were excluded.
CASI: Last year time frame
“Altogether, in the last year, how many (women/men) have you had sexual intercourse with?”	Number keyed in.
“Were there any (women/men) you had only oral sex with and never vaginal (or anal) sex?” If yes: “How many different (women/men) in the last year did you have only oral sex with and never vaginal (or anal) sex?” “Previously, you said you had sex with (previous answer inserted) (woman/women/man/men) in the last year. Does this number include the (woman/women/man/men) you had only oral sex with?”	Yes/no. Number keyed in. Response options: 1. Yes, (all/both) included. 2. No, did not include (all of them/both of them).
Face-to-face measures
“Tell me what your views are about the following sexual relationships:” “A married person having sexual relations with someone other than his or her partner?” “And what is your opinion about a person having one-night stands?”	Response options on a card provided to participants: 1. Always wrong 2. Mostly wrong 3. Sometimes wrong 4. Rarely wrong 5. Not wrong at all “Depends/Don’t know” not given on card but recorded by interviewer if participant gives this response. Note: So as not to exclude participants, individuals reporting Depends/Don’t know (269 for the first attitude and 641 for the second) were recoded to the middle option on the Likert scale (Sometimes wrong).