Is Availability of Porn the Key Predictive Factor in Incidence of Rape?

Commenter Joel got off on a tangent on a post last week, insisting that pornography was good for society because it correlated with decreased incidence of rape. He quoted a Scientific American piece from last year which said in part:

Perhaps the most serious accusation against pornography is that it incites sexual aggression. But not only do rape statistics suggest otherwise, some experts believe the consumption of pornography may actually reduce the desire to rape by offering a safe, private outlet for deviant sexual desires.

“Rates of rapes and sexual assault in the U.S. are at their lowest levels since the 1960s,” says Christopher J. Ferguson, a professor of psychology and criminal justice at Texas A&M International University. The same goes for other countries: as access to pornography grew in once restrictive Japan, China and Denmark in the past 40 years, rape statistics plummeted. Within the U.S., the states with the least Internet access between 1980 and 2000—and therefore the least access to Internet pornography—experienced a 53 percent increase in rape incidence*, whereas the states with the most access experienced a 27 percent drop in the number of reported rapes, according to a paper published in 2006 by Anthony D’Amato, a law professor at Northwestern University.

From this Joel concluded, "This doesn't mean that porn is good for any particular man, but all women should be greatful to live in a society where porn is widely available."

Obviously, even if there is a social correlation between porn availability and decreased rates of rape, this doesn't mean that use or creation of porn is moral. Lots of highly immoral activities may happen to correlate with (or even cause) decreases in other immoral activities, and the fact that one of these is immoral doesn't change the immoral status of the other.

However, the whole set of claims sounded fishy to me. "Lowest levels since the 1960s" seems like one of those statement which might, while technically true, still mask a huge difference, rather like "worst economic downturn since the Great Depression". Similarly, measuring statistics based on "the states with the least Internet access between 1980 and 2000—and therefore the least access to Internet pornography" seemed incredibly vague. The internet wasn't even particularly useful for pornography before the advent of the World Wide Web in the early '90s, so the 1980 to 2000 time frame intentionally included a lot of irrelevant time, and measuring the states with the least internet access was likely to simply get you the poorest and most rural states.

Further, I just found the whole proposed causal mechanism fishy. It sounded like the sort of thing where someone fished for a some correlations that worked just a little bit, but they were probably really just seeing some wider trend. My going hypothesis was the the rate of rape would mirror the rate of other violent crimes such as murder. If the rate of rape deviated from the rate of other violent crimes a lot, it would suggest that rape had different causal mechanisms that other forms of violent crime. If it rose and fell in a similar pattern (and I knew that violent crime as a whole had been falling since peaking in the early '90s) that would suggest that rape was just another, particularly nasty, form of social violence.

A little searching around led me to the Uniform Crime Reporting Statistics database maintained by the FBI. From there I pulled data on the rate per 100,000 of population of the set of violent crimes the database tracks: murder, aggravated assault, forcible rape, and robbery. The some total of these is the Violent Crime Rate. The following chart shows those four constituent rates since 1960.

[Click Image for Full Size Graph]

As you can see, the pattern match is very strong. The lowest correlation is between murder and rape, a 54% correlation. Robbery has a 84% correlation and aggravated assault has a 96% correlation. The overall violent crime rate has a 97% correlation with the rate of forcible rape. With correlations that high, if you ask me to tell you what the rate of rape is in the US in any given year, I'm not going to ask you, "Gee, how available was porn that year?" No, I'll ask you, "What was the overall rate of violent crime?" That is a far, far more predictive indicator than anything vague association with porn availability. And predictability is what science is all about. All the rest of what we're hearing is hand waving and self justification.

I then ran averages for each decade. In the 1960s the rape rate was 12.3 per 100,000 of population.

1970s, 26.0
1980s, 36.5
1990s, 38.3
2000s, 32.2

So Prof. Ferguson's statement is actually false, the rape rate is not at its lowest since the 1960s (it was lower in the '70s than it was in 2010 at 27.5) and the current rape rate is more than 2x the rate for the 1960s, when pornography was unquestionably much less available than now. The only way that this pseudo correlation comes to be is that the web has only existed since the early '90s and by coincidence all forms of violent crime have been on a steady decline since the early '90s. Unless one wants to claim that burglary and aggravated assault rates are being driven down by the availability of internet porn, we don't have much of a causal case to make here.

Joel later asked:

But your statement that the more porn => less rape correlation is "very loose" requires pushback. The fact is, those two trends have been observed together all over the world: whenever porn availability goes up, rape goes down. Everywhere. So:

1) You are actually telling us that this is coincidence? Really? It happens again and again, and you still say, "Coincidence"?

2) There's a simple and straightforward explanation as to why this is in fact causation: The safety valve effect of porn is real.


So tell me: what evidence would it take to convince you of this? Consider your answer carefully, because some researchers are probably looking into it right now, and you may rest assured that I will notify you when their results are published.

Given the shoddy nature of the "research" which Joel has quoted thus far, I rather doubt that there's any convincing research being done on the topic that would support Joel's pet claim, but I'll toss out a couple of minimum requirements to suggest that the availability of internet pornography has anything like a real effect on the incidence of rape:

• Get a large population sample at a young age and track their usage of pornography through regular surveys.  Show that sex crimes are committed significantly more often by those who do not use porn than those who do.  For bonus points, show that this is the result of lack of access rather than preference.  
• Track two regions which have shown similar trends in rape rate over the last 50+ years.  Significantly decrease the availability of pornography in one of these two regions, and then measure a statistically significant increase in the incidence of rape in that region relative to the other one, while their trends in other violent crimes remain the same.
• Do the reverse of the previous test: show two regions in which porn has been highly unavailable, and and in which their crime statistics have been highly similar.  Show that liberalizing porn availability in one of these regions correlates with a decrease in incidence of rape in that region relative to the other, while all other forms of violent crime remain on similar trends.
• Show that the previous two are repeatable in multiple areas.
• Show that the rate of rape deviates from the rate of other violent crimes in a way that can be explained by some sort of annual data on porn consumption.

Hit those criteria, and I'd think maybe a decent case is starting to be built.

I'll close by knocking the ball into the other court. The fact that the correlation between overall violent crime and rape is so incredibly high got me thinking: Is the rate of rape currently higher or lower than you would otherwise expect? I averaged out the overall violent crimes rates and rape rates from 1960 to 2010 and found that the ratio between these is 0.06. I then used that relationship to predict where the rape rate "ought" to be each year based on the violent crime rate. The result is that rape has actually declined more slowly than you would expect since 1996, based on the overall violent crime rate. it's actually a pretty steady "over trend" pattern for the last 10 years.



Can't imagine why that would be... Wait a minute. Those lines cross in 1999 and rape has been more common than the violent crime rate would suggest ever since (by an average of 9%). It hasn't been the case that internet pornography has been far more available in the years since 1999 than it was in the years before, has it?

(For the record: No, I don't think that's a particularly strong case. But it certainly cuts against the opposite contention which Joel is trying to make.)

UPDATE: In the comments, Joel has expressed grave concern that I used the FBI's Uniform Crime Reporting Statistics, which are based on crimes reported to law enforcement agencies (with appropriate adjustments to deal with apparent over or under reporting.) He avers it would be better to look at the National Crime Victimization Survey, which is based on an annual survey in which the Justice Department contacts 40,000 households and asks all members of those households over 12 what crimes they have been victims of that year. (The purpose of this methodology is to capture crimes which people don't report, for whatever reason.)

I don't think it's correct, as Joel does, to call these the "real" crime statistics, since the real number of crimes is fundamentally unknowable. Both the number of crimes reported to police and the number of crimes reported to DOJ surveyors are going to be subject to necessary sampling errors, and I'm not sure it's possible to say that one set of data actually reflects the true state of crime in the US better than the other. While it's certainly true that for a wide variety of reasons, women who are victims of rape may not report the crime to the police, it's also the case that in the survey some people will necessarily be missed (people living in highly marginal circumstances will be harder to contact), will not want to talk, or will be unavailable to talk (the NCVS only accepts data from the victim herself, which is why it reports no homicide data, this also means that cases of rape followed by murder will necessarily be unreported.)

All that said, there's no reason not to look at this data as well and see if it, like the FBI's data, suggest that the primary predictive factor for the rape rate is the rate of overall violent crime, or if there seems to be some other factor at work. Here's the overall trend chart.


Running the correlation between the sets of annual data for rape and total violent crime, I get a correlation of 89%. There's also a correlation of 91% between robbery and rape and of 91% between aggravated assault and rape. Simple assault shows a correlation of 82% with rape.

This means both that the rate of change in the rape rate, as shown in the NCVS's survey methodology is not unprecedented, and that it one can predict the number of rapes likely to have occurred in a year with a fair degree of accuracy by knowing the amount of overall violent crime. The availability of pornography does not have this predictive value -- or at least, none of these "studies" which attempt to show a connection between the availability of porn and the number of rapes even attempts to put together some sort of annual number which can be used to measure porn consumption or availability, and so any attempt at prediction is impossible.

Really the only thing that Joel's thesis has going for it, according to the NCVS's data is that the incidence of rape took a sharp down turn in 1991 which was three years before the overall violent crime rate took a sharp downturn in 1994. However, since that time the rape and violent crime as a whole have decreased at almost exactly the same rate.