BlackRock Inc., the world’s biggest asset manager, is asking companies with few or no women on their boards to explain themselves, saying that diverse groups make better decisions.

The firm’s investment stewardship group sent letters this week to all Russell 1000 companies with fewer than two female directors, asking that they justify how that aligns with their long-term strategies and to report on efforts to increase diversity on their boards. There are 367 Russell 1000 companies with fewer than two women on their boards, according to data compiled by Bloomberg.

“Irrespective of a company’s industry, location or size, we believe that a lack of diversity on the board undermines its ability to make effective strategic decisions,” BlackRock said in the letter signed by Michelle Edkins, global head of investment stewardship at the New York-based asset manager. The firm said a diverse board helps companies attract and retain employees.

BlackRock also updated its proxy voting guidelines, adding a stipulation that it expects companies to have at least two women directors on their boards.

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