Thursday, June 9, 2011

IQ and Getting Away With Murder

Let’s face it - criminals are usually stupid. The average IQ of white men doing jail time is well below the population average, and for those doing time for violent crime it’s even lower – 92 and 84 respectively. Why is that? Is it because the stupid are simply so much more likely to commit serious crime? Yes that is part of the explanation but it isn’t all of it.

For a start a fairly large fraction of crime goes unpunished. The national clearance rate for murder in the US is only about 62% right now. For lesser crimes it’s much lower. Furthermore the clearance rate has been dropping for some time in spite of better law enforcement technology. Part of the reason is rapid urbanization. In small rural areas where everyone knows everyone it’s much easier to narrow down the suspect list but in large urban areas it’s easy to stay anonymous. So since so many people are getting away with serious crime are the bright more likely to get away with it?

Murray and Herrnstein’s The Bell Curve shows a graph of the probability of being in the top 10% of self reported crime, by IQ (with the effect of social class controlled). It slopes slightly upward. Another line sloping downwards shows the probability of having been jailed by IQ. So, brighter men are both more likely to be in the top 10% of self reported crime, and less likely to have been jailed. Clearly brighter men do less jail time per unit of crime i.e. they are more likely to get away with it.

That shouldn’t be a surprise. Validity studies show that IQ has a very general validity. In brief, on any task requiring thought, almost all the variance in performance explainable by cognitive ability factors is explained by whatever it is that IQ tests measure. Usually this is called the g factor. Overcoming barriers when committing crime and making sure not to leave evidence that would arouse the suspicion of the police, requires definite problem solving. Higher IQs are better for successful problem solving.

Wouldn’t it be interesting to know your chances of being a successful Evil Genius? I mean just how hard would it be to be Professor Moriarty? To get an idea I had a go at putting numbers to this speculation. I produced a curve showing the probability of not doing jail time given that a man was in the top 10% of self reported crime. While this is interesting it’s more interesting to show the probability of getting away with a serious crime like murder because the police will be sure to at least try to solve it.

Unfortunately not many people admit to murder in self reported crime surveys, so I had to think of another way to estimate the odds of getting away with murder for various IQs. My approach involved serial killers and some creative statistics. It was lots of fun. (See the Appendix below for technical details.) You see the results in the graph below. To give some perspective I also included curves showing the probability of finishing high school, junior college, college and graduate school by IQ.

As you can see as a problem getting away with a single murder in the US is much like school or college in its intellectual demands. It’s just shy of junior college in difficulty. Being in the top 10% of self reported minor crime and avoiding jail time is shows a similar pattern but is closer to high school in difficulty. That suggests serious crime isn’t particularly demanding as intellectual tasks go. Being a successful serial killer i.e. getting away with 3 separate murders, is slightly tougher than a basic college degree but still a lot easier than obtaining a post graduate or professional degree.

Let’s try to define the minimum requirements for an Evil Genius IQ. I suggest two.

a) The same IQ as other geniuses, Nobel laureates say, which is an IQ of 146 on average.
b) Getting away with a murder at the standard 5% level of significance we apply to research results when we want to say it wasn’t due to chance. This implies a 95% chance of getting away with a murder and would require an IQ of 152.

A convenient compromise would be an IQ of 148 which falls neatly at +3 standard deviations above average and is neatly at the 1 in 1000 level. Combining this with the likelihood of someone with that IQ murdering someone in their lifetime, 1 in 8217 men, I estimate that there are only around 12 male and 1 female Evil Geniuses in the US today.


I assume that each killer has a unique and unchanging probability of avoiding being convicted for any murder. Call this E - for escape. I assume each murder is an independent event i.e. I don’t count mass murders. So the chance of getting away with n murders is Eⁿ, which is (1 – prob(getting caught)).

I reasoned that serial killers who managed to kill more victims before being caught are more talented at avoiding identification and capture, and that this talent is strongly related to their IQs. I also assumed a constant probability of being eventually caught applied to all serial killers. Initially I guessed that to be 0.62, the national murder case clearing rate.

So Eⁿ = 0.38 or
ln(E) = ln(0.38)/n and
E = exp(ln(0.38)/n)

Radford University’s Serial Killer Information Centre data base provided the IQ scores and number of victims for 48 white male serial killers in the US. Using these numbers and regressing IQ linearly on

E/(1-E) = m*IQ + c

I obtained an equation relating IQ to probability of getting away with a single murder. IQ did prove to be strongly correlated to victim number (r=0.44). I adjusted the constant c so that the equation would give me a probability of being caught of 0.62 for a single murder when IQ was the typical convicted murderer’s 84.

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