The share of female board members in the Russell 3000 index, which includes most public companies on major U.S. stock exchanges, increased to 20% in the second quarter of this year from 19% the previous quarter, according to Equilar Inc., a governance-data firm. When Equilar began tracking the measure in late 2016, 15% of board seats were filled by women.

The increases come amid calls for change from big investors and a new California law mandating female representation on public-company boards. In July, the last all-male board among S&P 500 companies—Dallas-based online car-auction company Copart Inc.CPRT -0.35% —added a female finance executive to its ranks.”