"Rethinking Timing of First Sex and Delinquency" 

 There was an interesting article this weekend in the Washington Post reviewing research on the relationship between the time at which adolescents become sexually active and subsequent anti-social behaviors (" Study debunks theory on teen sex, delinquency ", Nov. 11, 2007).  To sum up, existing research shows a strong and stable correlation between early "sexual debut" and delinquency later in life.  This has often been interpreted as a causal relationship by policy advocates, despite the obvious potential for confounding.  It seems clear that unobserved characteristics - thrill-seeking, risk-taking preferences (or even a simple lack of adult supervision) - would encourage both early sexual activity and delinquency. 

 The WaPo article contrasts these existing results with a new study by researchers at the University of Virginia, who look at differences in the timing of sexual debut and delinquency among pairs of twins.  As the Post reports, "Other things being equal, a more probing study has found, youngsters who have consensual sex in their early-teen or even preteen years are, if anything, less likely to engage in delinquent behavior later on."  This is a fairly accurate and measured interpretation of the results of the paper, which is worth commending since we often give the media a hard time on this blog for over-selling the results of scientific papers.  (Now if we can just get them to link to the papers from their website, as the New York Times does fairly regularly.)  
 


 The authors of the twin study are somewhat more ambitious in their claims. Here is the abstract to the paper: 

 
 Rethinking Timing of First Sex and Delinquency 

 K. Paige Harden , Jane Mendle, Jennifer E. Hill, Eric Turkheimer and Robert E. Emery 

 (1)  Department of Psychology, University of Virginia, Charlottesville, VA 22904-4400, USA  

 Abstract  The relation between timing of first sex and later delinquency was examined using a genetically informed sample of 534 same-sex twin pairs from the National Longitudinal Study of Adolescent Health, who were assessed at three time points over a 7-year interval. Genetic and environmental differences between families were found to account for the association between earlier age at first sex and increases in delinquency. After controlling for these genetic and environmental confounds using a quasi-experimental design, earlier age at first sex predicted lower levels of delinquency in early adulthood. The current study is contrasted with previous research with non-genetically informative samples, including Armour and Haynie (2007, Journal of Youth and Adolescence, 36, 141–152). Results suggest a more nuanced perspective on the meaning and consequences of adolescent sexuality than is commonly put forth in the literature.  
  
In the text of the paper, the authors go further: 
  
The current study suggests that there may be positive functions for early initiation of sexual activity, in that the co-twin with earlier age at first sex demonstrated  lower  levels of delinquency in early adulthood 
  
The Virginia paper makes a convincing case that we should question the existing research, but I think they are underselling the threats to their own inferences. 

 Twin studies have been quite influential in a number of areas, and they have many benefits; they allow for balance on genetic and common environmental characteristics that would be exceedingly difficult to achieve in a typical observational study.  Moreover, studies comparing idential and fraternal twins at least offer the possibility of teasing out the effects of genetic and environmental factors.  At the same time, as twin studies move from biomedical to behavioral questions, there are some issues that deserve further consideration.  

 The first of these problems is selection within the sets of twins.  If there is one thing that we know, it is that sex involves selection.  Moreover, not to put to fine a point on it, but one of the parties involved in that selection process is choosing between twins (in many cases, identical twins!).  The fact that the non-twin partner chose one twin over the other suggests that unobserved differences between the twins play an important role.  The authors allude to this problem and describe their results as ``quasi-causal'', but they may be underestimating the importance of these ``uncontrolled confounds'' given the non-random character of the assignment process.  Focusing on twins to achieve balance on genetic and shared environmental characteristic may end up increasing the overall bias of the estimates by increasing the imbalance in the unobserved unit-specific characteristics. 

 The second, and in my opinion more interesting, problem with the study is that it doesn't take into account the interaction within each set of twins.  In effect, the researchers are conflating two treatments: the timing of each subject's sexual debut and the timing of the sexual debut for each subject's twin.  This suggests an interference problem, because the delinquency outcome for subjects may depend on whether they became active before or after their twin did.  One could easily imagine a scenario in which one twin becomes active and the other twin responds by acting out due to frustrations of one sort or another.  In that case, it isn't so much that earlier sexual activity has a "positive function" for the twin engaging in it but rather a "negative function" for the twin that is not active; this is something that the data cannot answer.  

 I think that this is a general problem when using twin studies to estimate the effects of behavioral treatments.  Some treatments will have a greater effect on the untreated twin that others, and my guess is the more the treatment is in the realm of social science, the more we should worry about these issues.  At the very least, we should be skeptical about how the estimates obtained from twin studies would generalize to the population at large given the inherent interference problems.