State Street Global Advisors Inc. is urging public companies to disclose more concrete data on their racial diversity or face pushback from the asset manager in proxy meetings.

SSGA is one of the world's largest asset managers, with more than $3 trillion in assets under management. Starting in 2021, the firm will ask companies in its investment portfolio to articulate their risks, goals and strategies related to racial and ethnic diversity and to make relevant disclosures available to shareholders.

"As always, our primary tool is engagement with management and the board with the objective of understanding a company's plan and how the board is carrying out its oversight role," SSGA Global Chief Investment Officer Richard Lacaille wrote in an Aug. 27 letter to public board chairs. "However, if required, we are prepared to use our proxy voting authority to hold companies accountable for meeting our expectations."

The letter outlined five key areas where SSGA expects U.S. companies in its portfolio — and "to the greatest extent possible," non-U.S. companies — to provide specific communications: Companies must articulate the role diversity plays in their broader human capital management practices and long-term strategy. They must describe their diversity goals, how these contribute to the company's overall strategy, and how progress on these goals is managed and measured. SSGA is also seeking metrics on race, ethnicity and gender diversity in companies' boardrooms and overall workforce. The asset manager also wants to know how a company's board reflects the diversity of its workforce, community, customers and other key stakeholders. And companies must describe how their board executes its oversight role in diversity and inclusion.

Big investors are paying increasing attention to environmental, social and governance issues like diversity. BlackRock Inc. and SSGA have urged their portfolio companies to promote women and minorities within their ranks and increase gender diversity on their boards. But disclosure remains a problem when it comes to understanding diversity at companies. Many diverse attributes, like sexual orientation and ethnicity, are impossible to identify if a company does not self-report.

Lacaille acknowledged the difficulty of measuring race and ethnicity in a uniform way, but said it is critical for boards and investors to have more robust diversity information on these topics.

"As long-term investors, we are convinced that the lack of racial and ethnic diversity and inclusion poses risks to companies that senior managements and boards should understand and manage," he wrote.