No one may have a more intimate and wary reading of the unstable behavior of a potentially dangerous gun owner than his own family. This is the driving force behind a new California law that, for the first time in the nation, allows concerned family members to petition a court to seize the firearms of a family member they fear is on the verge of harming himself or others.

The law was prompted by the murderous spree near Santa Barbara, Calif., in May, when a troubled 22-year-old, Elliot Rodger, killed six people and himself and wounded 13 others. A few weeks earlier, his mother had warned authorities that his behavior was increasingly erratic, even life-threatening. But he was able to convince police that he presented no threat, even though he had legally purchased three guns and devised plans for a rampage.

The California law cleverly confronts gun rights advocates whose customary response after shooting sprees is to try to get people to focus on complex mental health issues so as to divert attention from the need for gun safety legislation. The California approach embraces both fronts. Why shouldn’t family members be able to have lethal weapons confiscated on the basis of threatening behavior they can attest to firsthand? Any defense from the gun owner will be weighed by the court. Besides helping to prevent mass shootings, the law could help families prevent suicides in their midst.

The law, similar to one authorizing restraining orders in domestic violence cases, is not a total cure. Mr. Rodger used a knife to kill his first three victims, then guns for the rest. But the Legislature’s Democratic majorities wisely decided some progress against the gun scourge was possible by inviting relatives to weigh in with valuable and timely family expertise on those who seem near the breaking point and who also have guns legally at hand.