The effectiveness of sex education may be down to the number of girls in a class - and the humour and discipline of the teacher.
As HMI prepares to release its findings on the implementation of the McCabe report on sex education and the Catholic Church continues to argue with the Scottish Executive on the issue, research by Glasgow University academics warns that "pupil discomfort in sex education lessons is high".
They suggest the answer, based on interviews with 59 teachers and observations of 62 lessons in 25 east of Scotland secondary schools, may be - girls. Only 10 of the lessons, which were with third and fourth-year pupils, were classed as having a high level of participation.
"Classes with the highest level of participation," the study states, "tended to be comprised predominantly of girls, were older and were headed by a teacher with a strong sense of humour, a tight disciplining style and who was new to the class that year."
Material the pupils appeared to find interesting and teaching methods they were comfortable with were also effective. The best results were seen in schools with a mixed catchment.
The research was carried out by Katie Buston and Daniel Wight of the Medical Research Council's social and public health sciences unit at Glasgow University. It is reported in the current edition of the journal Sex Education.
Their report notes that sex education in mixed-sex classes poses problems for both boys and girls. "Although girls tend to be interested in sex education, they often feel threatened and ill at ease, and fear of ridicule from the boys prevents them from participating.
"It is, therefore, unsurprising that classes composed predominantly of girls were among the most participative observed."
While this appears to point towards the benefits of single-sex classes for sex education, strong arguments remain for mixed-sex classes in helping to promote discussion between the sexes. But, they add, that depends on levels of comfort in the class.
The researchers also draw attention to the fact that fourth-years found it easier to take part in lessons than those in third year, challenging the notion that the younger the better for sex education. Their report suggests that lessons should be "targeted" at different groups.
The oft-observed view that pupils are drawn to teachers with a sense of humour (see page three) receives a mixed reaction in the research.
Successful lessons were largely delivered by teachers with a strong sense of humour - but they presided over less successful lessons as well.
"A few jokes cracked by a teacher are not going to make a class participate instantly," the report states. "Humour is not sufficient to foster full participation and, of course, it may be that a teacher feels more able to be light-hearted when his her class is throwing themselves into the work at hand.
"However, fun sex education can make pupils more confident and more likely to participate."
A disciplined approach was a more consistent pointer to success and this was present in all highly effective classes, the research found. Pupils valued the "protection" of teachers who ran a tight ship, particularly in cracking down on off-task comments or ridicule directed at individuals.