The point I wanted to make in the previous post is this. Ignore pedophilia for the moment, and think of high cholesterol instead. Now presume that 50% of men have high cholesterol. A good diagnostic test for high cholesterol would then have to improve upon this base rate. Simply claiming that every man has high cholesterol, would at the very least, be 50% accurate.

The medical field, nevertheless, could still conclude that a good diagnostic test isn’t needed because the cost of misdiagnoses, either a false positive (claiming that a man has high cholesterol when in fact he doesn’t) or a false negative (claiming a man does not have high cholesterol when in fact he does) is trivial because statins do no harm. If a statin is consumed, but not needed, so what; if a statin is consumed, and then lowers cholesterol, great.

The financial and emotional costs of misdiagnoses, let alone the health risks, are another matter entirely. This is why the field of medicine puts a high priority on good diagnostic tests.

Returning again to pedophilia, if only 5% of men are pedophiles, it’s impossible for a psychological test to improve upon this base rate. Because psychology, unlike medicine, often fails to put a premium on good diagnostic tests, sufficing instead with makeshift self-reports, the problems are then compounded. Existing tests for pedophilia, in fact, have atrocious construct validity, relying as they do upon small, truncated samples of convicted pedophiles for validation.

Much worse however is the fact that a poor diagnostic instrument in the field of pedophilia is going to make countless errors (false Positives and false Negatives) that are going to have grave consequences where litigation is concerned. True pedophiles (false negatives) are going to escape conviction and gain custody of their kids.