Causation and Kellermann

circe wrote:

In homes with guns the homicide of a household member is three times
more likely to occur than in homes without guns. New England Journal
1993;329.1084-1091.

David Friedman writes:

You may also have noticed that the death rate in hospitals is much
higher than in hotels. So if only we abolished the hospitals … .

But the study controlled for a variety of factors such as age, sex,
neighbourhood, criminal record, drug use etc etc. If take your
hospitals/hotels analogy and control for severity of disease/trauma we
would find that hospitals are safer. For example, if you just looked
at people suffering severe heart attacks, we would find them more
likely to survive if they were in a hospital, rather than a hotel.

In other words, a statistic like that assumes that the causation runs
“have a gun in the home, therefore more likely to be killed.” But an
alternative, perhaps more plausible, causation is “Be in a situation
where you are at risk of being killed, therefore have a gun in the home.”

This alternative is less plausible when you look at the stratified
analysis in Kellermann’s study. Gun owners were not at any greater
risk of non-gun homicide, only of with-gun homicide. It does not seem
plausible that people would only obtain guns to defend against threats
from gun-armed enemies and not also arm themselves against an enemy
that carries a knife.

Similarly, gun owning households were only at greater risk of homicide
from intimates (spouse, lover, family) i.e. those people likely to
have access to the household’s gun, and not at greater risk of
homicide from outsiders. It does not seem plausible that people would
only arm themselves against threats from inside the household and not
from outside.