Alison Parkes, of Glasgow University's Medical Research Council social and public health sciences unit, led a research study which looked at how the effects of substances and social pressures influenced a teenager's decision not to use condoms.
The team found the main reasons were psycho-social and the effects of alcohol or drugs at the time of intercourse. Between 1996 and 1999, the study questioned 14 to 16-year-olds at 25 schools in Tayside and Lothian.
Researchers found 22 per cent of 14-year-olds smoked, drank or took drugs regularly. Of sexually active 16-year-olds, 10 per cent had had four or more partners in a year, and 40 per cent did not use a condom. Girls, and those from poorer backgrounds, were more likely not to use condoms and to smoke regularly at 14, and boys more likely to use cannabis.
The researchers felt they needed to understand why having a greater number of partners and having sex more often should be associated with lower condom use, suggesting it may be to do with "sensation-seeking".
"Among adolescent substance users, being drunk or stoned at intercourse was only one factor related to not using condoms. Psycho-social factors and having more sexual partners also explained substance users' condom use."