Though it’s shockingly common to get groped on the subway, it’s hard to know what to do about it. This is particularly true in Japan, where two-thirds of women say they’ve been groped according to a 2004 survey, though only an estimated 10% report the incidents.

Now, women in Japan have a new way to penalize their assailants: by stamping them with invisible ink.

In August, a Japanese writing instrument and stamp company Shachihata released its anti-groping stamps in Japan. The 500 stamps sold out in less than 30 minutes, CNN reports, a testament to the severity of the country’s sexual harassment problem.

This isn’t the only way Japanese women have tried to protect themselves. Japanese trains have had women-only cars for more than a century. Earlier this year, a startup issued an app that screams “Stop it!” at would-be harassers. Clearly, though, the problem still persists, justifying the demand for the recently-issued stamps.

An invisible ink stamp to mark assailants is a positive gesture to promote gender equality, but it’s limited. There are no consequences for a person whose hand is stamped, and no regulators or police are checking for it. Though some people think the stamps might deter groping, it’s possible that the act of stamping someone could have the opposite effect, perhaps causing them to respond defensively or even violently, as one man did in 1988 after he was accused of groping.

A law or regulation—especially one that is well enforced—that protects women from sexual harassment on crowded trains would be more effective than relying on women to defend themselves. According to the Japan Times, men are not legally prohibited from using women-only cars and most women don’t speak up because they fear not being believed or arriving late to work.

Image: OECD

Japan has the lowest ranking in gender equality among G7 countries, but its government has been making legal pushes to change that. In 2016, the country passed legislation to increase parental leave to 12 months and eradicated a tax deduction for dependent spouses to motivate women to work. In 2017, to encourage Japanese women to have more children, the federal government approved massive subsidies to make daycare more affordable or free.

However, laws cracking down on harassment haven’t received any such update. While it is illegal to sexually harass someone in Japan, victims can be hesitant to come forward because of the difficulty in proving the harassment happened, and because they fear retribution. Current rape laws in Japan do not protect victims who do not fight back against sexual violence or cannot prove they were incapable of resistance. As a result, women are forced to take matters into their own hands—even if that means fighting against would-be assailants with invisible stamps.