We are told a recent US study found that sexually active teenage girls are more likely to suffer depression, and to attempt suicide, than girls who are not sexually active. Ms Albrechtsen portrays the study as hard evidence that sexual activity causes depression and suicide.
This conclusion cannot be drawn from the data. Correlation is not causation. We may as well conclude that depression causes sexual activity; or that goannas prevent overpopulation, because smaller towns tend to contain more goannas.
There is a host of alternative explanations for the US data. A subgroup of teenagers might have higher rates of sexual activity and depression, giving rise to a correlation, without sexual activity causing depression in any individual. The data are based on survey responses, and young people who are reticent about admitting to sexual adventure on the questionnaire might also deny having suicidal thoughts; and so on. The authors of the report cited by Ms Albrechtsen were circumspect enough to acknowledge alternative explanations, and to investigate some of them.
A lot of the rhetorical strength of Ms Albrechtsen's argument rests on invoking the report as "evidence", with the implication that it is objective and authoritative. The source of the report is the Heritage Foundation, described on its website as "a think-tank whose mission is to formulate and promote conservative public policies". As far as we know, this report has not passed through scientific peer review. It does not deserve to be represented as objective evidence.
Adrian Baddeley
Professor of Statistics
University of Western Australia