More On The Danish Childlessness-Is-Death Study

Which I have now read, thanks to very generous Santa Claus/Father Christmas readers.  The report on the study is a very short one.  It only lists the final figures, not the stages of getting to them.  But what is shown looks OK to me, i.e., I don't have to add study criticisms to the earlier popularization criticisms.

Still, I learned something important from reading the report itself:  The manner in which its authors explained the psychiatric illness findings.  Several popularizations mentioned them:

This site gives us false (well, incomplete)  information:

The study also examined the mental health aspects of failing to conceive and found that childless couples were twice as likely to suffer mental illness as those who adopted children.

The study found no statistically significant differences in mental health aspects (with the possible exception of addiction) between those who had a biological child through the fertility treatment and those who failed to have one.  That statement should have compared the adopting group to both those who had a biological child and those who did not but didn't adopt.

This site gets the basic numbers right:

And while the study didn’t show any difference in psychiatric illness between people who had children using IVF and those who did not, the parents who ended up adopting children were half as likely to be diagnosed with mental illness.

It was the adopting group which did better than the other two groups in terms of mental health.

But that doesn't really make much sense as a part of a hypothesis about the impact of children on one's mental health-being, unless having biological children is somehow bad or neutral for the parents' mental health, while adopting is good.

And indeed, the authors explain that the finding might be caused by the way adopting parents in Denmark are screened or by other factors not related to the basic thesis of the study:

The rate ratios in parents who adopt are surprising. Perhaps this is related to survival bias, as Danish adoption regulations mandate that a fertility treatment must be completed before an adoption process can begin. Alternatively, parents who adopt could be a selected group, as prospective parents’ health and economic condition are evaluated by the National Adoption Board. The rate of adoption is higher among affluent parents, 1.75 (1.46–2.08), but unrelated to the Charlson comorbidity index (p=0.93). Thus, the event of becoming an adoptive parent may not be exogenous given the covariates and the assumption for the natural experiment may be violated.
Our analyses show that childlessness has little impact on the rate of psychiatric illness. This lends support to the conjecture that previous reports of higher risks in the childless8–10 may actually reflect reverse causality, as individuals with an undiagnosed or insidious psychiatric illness are more likely to be childless and, subsequently, diagnosed with a psychiatric illness. However, our results show that individuals who adopt are at lower risk of psychiatric illness. It is likely that this lower rate is an artefact as mentioned above. Consistent with previous studies, our results suggest that the rate of substance use is higher in the childless.

Why bother writing about this?  Because those tiny sentences in the popularizations will float around inside people's heads, and having facts wrong is not that good.

Also, I'm aiming to bore away all remaining readers.