This is a good example of neglecting magnitudes of effects. I think in this case most people just don’t know the magnitude, and wouldn’t really defend their answer in this way. It’s worth considering why people sometimes do continue to emphasize that an effect is not literally zero, even when it is effectively zero on the relevant scale.
I think it’s particularly common with risks. And the reason is that when someone doesn’t want you to do something, but doesn’t think their real reason will convince you, they often tell you it’s risky. Sometimes this gives them a motive to repeat superstitions. But sometimes, they report real but small risks.
I’ll ignore the associations with mental illness, which are known to be the result of confounding, although this is itself an interesting category of fake risks. For example a mother that doesn’t want her child to get a tattoo, because poor people get tattoos, could likely find a correlation with poverty, or with any of the bad outcomes associated with poverty.
Let’s focus on testicular cancer, and assume for the moment that this one is not some kind of confounding, but is actually caused by smoking marijuana. The magnitude of the association:
The strongest association was found for non-seminoma development – for example, those using cannabis on at least a weekly basis had two and a half times greater odds of developing a non-seminoma TGCT compared those who never used cannabis (OR: 2.59, 95 % CI 1.60–4.19). We found inconclusive evidence regarding the relationship between cannabis use and the development of seminoma tumours.
Testicular cancer is not common: about 1 of every 250 males will develop testicular cancer at some point during their lifetime.
So while doubling your testicular cancer risk sounds bad, doubling a small risk results in a small risk. I have called this a “homeopathic” increase, which is perhaps unfair; I should probably reserve that for probabilities on the order of homeopathic concentrations.
But it does seem to me to be psychologically like homeopathy. All that matters is to establish that a risk is present, it doesn’t particularly matter its size.
Although this risk is not nothing… it’s small but perhaps not negligible.
This is a good example of neglecting magnitudes of effects. I think in this case most people just don’t know the magnitude, and wouldn’t really defend their answer in this way. It’s worth considering why people sometimes do continue to emphasize that an effect is not literally zero, even when it is effectively zero on the relevant scale.
I think it’s particularly common with risks. And the reason is that when someone doesn’t want you to do something, but doesn’t think their real reason will convince you, they often tell you it’s risky. Sometimes this gives them a motive to repeat superstitions. But sometimes, they report real but small risks.
For example, consider Matthew Yglesias on the harms of marijuana:
I’ll ignore the associations with mental illness, which are known to be the result of confounding, although this is itself an interesting category of fake risks. For example a mother that doesn’t want her child to get a tattoo, because poor people get tattoos, could likely find a correlation with poverty, or with any of the bad outcomes associated with poverty.
Let’s focus on testicular cancer, and assume for the moment that this one is not some kind of confounding, but is actually caused by smoking marijuana. The magnitude of the association:
What we really want is a relative risk (how much more likely is testicular cancer among smokers?) but for a rare outcome like testicular cancer, the odds ratio should approximate that. And testicular cancer is rare:
So while doubling your testicular cancer risk sounds bad, doubling a small risk results in a small risk. I have called this a “homeopathic” increase, which is perhaps unfair; I should probably reserve that for probabilities on the order of homeopathic concentrations.
But it does seem to me to be psychologically like homeopathy. All that matters is to establish that a risk is present, it doesn’t particularly matter its size.
Although this risk is not nothing… it’s small but perhaps not negligible.