There’s no reason a priori to suppose that any positive or negative effects not currently priced will be of the same order of magnitude.
There are some a posteriori reasons though—there are numerous studies that reject a causal link between the number of firearms and homicides, for example. This indicates that firearm manufacturers do not cause additional deaths, and therefore it would be wrong to only internalize the negative costs.
If you think there are benefits to having a population where most people own guns that are not going to be captured by the incentives of individuals who purchase guns for their own purposes, it’s better to try to estimate what that effect size is and then provide appropriate incentives to people who want to purchase guns.
That’s not true. It is not better, because providing appropriate incentives is very likely impossible in this case, e.g.: - due to irrational political reasons (people have irrational fear of guns and will oppose any efforts to incentivize their purchase, while supporting efforts to disincentivize it); - due to the fact that a reward system for preventing crime can be easily gamed (cobra effect), not to mention the fact that it will probably be very costly to follow up on all cases when crime was prevented; - due to the fact that positive outcomes of gun ownership are inherently hard to quantify, hence in reality they will not be quantified and will not be taken into account (McNamara fallacy).
There are some a posteriori reasons though—there are numerous studies that reject a causal link between the number of firearms and homicides, for example. This indicates that firearm manufacturers do not cause additional deaths, and therefore it would be wrong to only internalize the negative costs.
That’s not true. It is not better, because providing appropriate incentives is very likely impossible in this case, e.g.:
- due to irrational political reasons (people have irrational fear of guns and will oppose any efforts to incentivize their purchase, while supporting efforts to disincentivize it);
- due to the fact that a reward system for preventing crime can be easily gamed (cobra effect), not to mention the fact that it will probably be very costly to follow up on all cases when crime was prevented;
- due to the fact that positive outcomes of gun ownership are inherently hard to quantify, hence in reality they will not be quantified and will not be taken into account (McNamara fallacy).