Promiscuity in US high schools is at a twenty year low and teen birth rates have fallen by over one third from 1991 through to 2009. So why is it if teens are sexually active at lower frequencies, and appear to be taking more precautions, that 50% of new STI infections last year were in people younger than 24?
In my last post we talked about the theory that “More Sex is Safe Sex”, but there has to be a better explanation for why STI rates for teens are persistently high beyond simply saying that too many teen boys are having sex with too few teen girls. There is, and it has to do with both changes on the teen sex “market” on both the intensive and extensive margin.
Image in a school where there are 100 students, of which 50 are sexually active with other students in the school. It doesn’t matter what their gender is, nor does it matter if they are heterosexual or not, what matters is that they are not in monogamous pairs; they are teenagers after all and so each one can have several sexual partners over the course of a year.
At the beginning of the year one student arrives infected with an STI. Given that students are not in committed relationships, that student infects his/her partner, who then infects his/her next partner and so on. By the end of the year a certain percentage of students body has become infected with the disease – presumably less than half since only 50% are sexually active.
The following year, another 100 students arrive at the school but now only 40 students are sexually active. Again, at the beginning of the year, a student arrives with an infection and the whole process starts all over again. Now given that there are less sexually active students we might be tempted to predict that fewer students are infected by the end of the year. But, that prediction would probably be wrong.
The problem is that not all students behave in the same way and chances are that the students who have chosen to not have sex are the students who, had they been sexually active, would have taken the greatest precautions – they are the students who are the most risk averse.
Remove the most precautious students from the teen sex market and not only will the infection rate not go down, but it could very well increases. This is because those who might have found themselves with precautious sex partner (for example, insisting on condom usage) in they were sexually active are now left with partners who are more risk neutral when it comes to safe sex – or even risk loving.
That is the change on the extensive margin – otherwise safe players have left the market.
The change on the intensive margin is a little more complicated.
A recent NBER working paper suggests that in schools in which girls outnumber boys, the gender imbalance increases the willingness of girls to participate in relationships that involve sex.
For example, they find that the fraction of grade 12 girls who are in a relationship and having sex is much higher than the fraction who state that they would prefer to be in a relationship without sex. The fraction of grade 9 girls who are in a relationship and having sex, on the other hand, is very similar to the fraction that state that that is their preference.
They also find that the fraction of grade 12 boys who are in a relationship and having sex is very similar to the fraction who state that this is their preference. And that the fraction of grade 9 boys in relationships and having sex is much lower than the fraction who state that they would prefer to be in a relationship that involved sex.
These results suggest that in grade twelve, boys have greater bargaining power over sexual activity because they are having as much sex as they want, and the girls are having more than they want. In grade nine, the opposite is true – girls have greater bargaining power over sexual activity because they are having as much sex as they want, and the boys are having less than they want.
This is relationship exists because grade twelve girls have to compete for scarce grade twelve boys; not only because boys less likely to finish high school but also because grade twelve girls have to compete with girls in younger grades who are happy to have relationships with older boys. This “market power” of older boys not only puts pressure on girls to have relationships that involve sex (even when they would prefer that they didn’t), but hands bargaining power over to boys when it come the decision to consistently applying protection against disease (i.e. wearing a condom).
Since 1992 the share of girls who are sexually active has fallen five percentage points (from 51% to 46%) but the share of boys who are sexually active had fallen by far more – 11 percentage points (from 57% to 46%). That isn’t a problem for girls who don’t want to have sex, in fact that should make it easier for them to find a boy with the same preferences. But for girls who do want to be sexually active this increase in the market power of boys is bound to lead to lower rates of condom use.
Add to this the fact that older boys are not only transmitting infections to girls in their own year, but also to girls in the cohort below them, and you have the making of an epidemic as diseases are passed through the “generations” of students.
To be complete we really need to consider the lack inter-racial dating in high school. The fact that Black teenage girls are 16 times more likely to be infected with chlamydia, 7 times more likely to be infected with gonorrhoea and 28 times more like to be infected with syphilis than White teenage girls illustrates that over sight in this theory.
But that, I am afraid, will have to be a discussion for another day.
Peter Arcidiacono, Andrew W. Beauchamp, and Marjorie B. McElroy (2010). “Terms of Endearment: An Equilibrium Model of Sex and Matching.” NBER Working Paper No. 16517