Yet right-to-work laws still threaten to expose real weaknesses inside unions: namely a lack of solidarity and participation among members. Twenty-five years ago, in their study on union membership attitudes and participation, Daniel Gallagher and George Strauss wrote that "compared with European unionists, those in North America look upon unionism more as an insurance policy than an instrument in the class struggle or even as a social movement." Labor's approach to its membership has changed little during the intervening years, with unions still presenting themselves as a service to their members. Though it is difficult to gauge levels of solidarity, one way of measuring it is through the use of strikes. Strikes are among labor's strongest weapons, but they require a great deal of solidarity to ensure that workers don't cross the picket line or that the union does not face a decertification vote following the strike. Between 1990 and 2015, the number of strikes declined by more than 90 percent, from 801 in 1990 to 72 last year.