If you personally know somebody who died from X, then X is dangerous.

This risk assessment heuristic is not as stupid as it sounds.

Important clarification: celebrities and friends of friends do not count, only people that you knew personally do.

Let’s start by quantifying what dangerous is here. A typical human can keep track of about 150 people (the Dunbar’s number). Assuming none of those people died from X and average age is 40 years (half of life expectancy), probability of death from X is less than 3/(150·40) = 5·10-4 with 95% confidence (using rule of three). For comparison, fatality rate among logging workers or fishermen is about 10-4 per year.

Jared Diamond gives this example:

“I first became aware of the New Guineans’ attitude toward risk on a trip into a forest when I proposed pitching our tents under a tall and beautiful tree. To my surprise, my New Guinea friends absolutely refused. They explained that the tree was dead and might fall on us.

Yes, I had to agree, it was indeed dead. But I objected that it was so solid that it would be standing for many years. The New Guineans were unswayed, opting instead to sleep in the open without a tent.

I thought that their fears were greatly exaggerated, verging on paranoia. In the following years, though, I came to realize that every night that I camped in a New Guinea forest, I heard a tree falling. And when I did a frequency/risk calculation, I understood their point of view.”

This works the other way, too. If none of the people you knew were killed by a shark but two died from heart attack, maybe you should worry less about shark attacks and instead do something to lower the risk of heart attack.