The study has all the hallmarks of a carefully rigged anti-gun "study". The most interesting thing about the study is that apparently it is possible for people to gain access to the entire list of handgun transactions by name for the state of California. That's a surprise to me, because what other legally required registry of people's private activity is readily available from the government?
So now we all know that any government database of firearms transactions ends up leaking out to people who can then connect that data to whatever other data they want to connect it with.
As for the study itself it claims to track the relative risk of living with someone who purchases a handgun. It does not include any data related to illegally owned firearms, or firearms not in the California registry the researchers used.
The study claims that rates of homicides are higher for people living with people who have firearms, but does not attempt to control for all of the factors which may lead to the results they claim. I suspect further analysis of the study, or access to the underlying data, will show that their study methodology is flawed.
The CA govt collects and willingly disseminates lawful firearms owner data. Of course, only law abiding persons are represented in the data as households with unregistered handguns aren’t nor can be known.