The Dangerous Economist: Self-defense laws and Type I & II Errors

Wednesday, November 05, 2025

Self-defense laws and Type I & II Errors

See Six Words Every Killer Should Know: ‘I Feared for My Life, Officer’: The number of legally sanctioned homicides has grown substantially in states with expanded self-defense rights under stand-your-ground laws by Mark Maremont and Paul Overberg of The WSJ.

I used the book The Economics of Public Issues in my micro classes. Chapter 1 is called "Death by Bureaucrat." It discusses how the Food and Drug Administration can make either a Type I error or a Type II error.

Type I error: The FDA approves a drug before enough testing is done and when people take it, there are harmful side effects.

Type II error: The FDA tests a drug longer than necessary to stay on the safe side. But people might suffer because the drug is not yet available. 80,000 people died waiting for Septra to be approved.

The FDA would usually rather make a Type II error because the public can blame the FDA if a Type I error occurs.

Something similar is happening with self-defense laws. It has gotten easier to claim that you killed someone in self-defense. What about the times when there is not enough evidence one way or another to know if the killer is telling the truth?

This is where Type I & II Errors come in. We could make it very easy to claim self-defense. Then some people who did not kill in self-defense would not be charged or convicted. That would be a Type I error. Some real killers would go free so we could make sure that innocent people never went to jail.

We could make it very hard to claim self-defense. Then some people who did kill in self-defense would be charged or convicted. That would be a Type II error. Some innocent people would go to jail to make sure that no killers ever went free.

Excerpts from the article:

"It’s easier than ever to kill someone in America and get away with it.
In 30 states, it often requires only a claim you killed while protecting yourself or others."
"so-called stand-your-ground laws in those 30 states extend legal protections to public places and make it difficult for prosecutors to file homicide charges against anyone who says they killed in self-defense."
"Justifiable homicides by civilians increased 59% from 2019 through 2024 in a large sample of cities and counties in those states, the Journal found, compared with a 16% rise in total homicides for the same locales."
"A retired Las Vegas police officer walked free after fatally shooting a retired computer network engineer during a dispute over who had the right of way in a Walmart parking lot. Both men got out of their vehicles. Both were armed. The ex-officer said the retired engineer pointed a gun at him first."
"In many stand-your-ground cases, authorities are left to rely on the word of survivors. The laws are written to protect those who tell authorities they feared for their life."
Self-defense laws across the U.S. have long varied. In many states, people faced with an aggressor had the responsibility to retreat, if possible. The general exception fell under the so-called castle doctrine: A resident could legally kill a home intruder who posed a serious threat without retreating.
Under stand-your-ground laws, a person no longer had the duty to retreat in any place they were legally allowed to be. They could defend themselves and others with lethal force if they reasonably thought they were in danger of death or serious harm."
"A provision in Florida’s law, since mirrored by about 15 others, added a presumption that, in most cases, a killer claiming self-defense acted reasonably, requiring prosecutors prove otherwise."
"Homicides in stand-your-ground states were nearly twice as likely to be deemed justifiable as they were in other states in 2024, the Journal found.
Supporters of stand-your-ground laws have often argued that the measures would mostly be used by people to protect against armed strangers. Nationally, FBI data shows about 60% of reported self-defense killings involved a family member, boyfriend, girlfriend, friend, neighbor or acquaintance, according to a tally of 2019-2024 cases that recorded such relationships."