Apple (AAPL) CEO Tim Cook announced he was “proud to be gay” on Thursday, becoming the first leader of a major company to come out publicly.

Cook, in an essay published by Bloomberg Businessweek, said he hoped his disclosure would help other people feel less alone and inspired to insist on their equality. The Apple leader had spoken out in favor of equal rights for gay, lesbian, bisexual and transgender people in a speech at the Alabama state capitol on Monday without revealing his own orientation.

In the essay, Cook said he was setting aside his preference for privacy so he could “know that I’m doing my part, however small, to help others. We pave the sunlit path toward justice together, brick by brick. This is my brick.”

Apple has been a leader in the movement to grant equal rights to LGBT people. And in corporate America, the movement has seen broad success. Among Fortune 500 companies, 91% provide explicit protections on the basis of sexual orientation and 61% on the basis of gender identity, an historic high, according to a survey by the Human Rights Campaign. Two-thirds of companies offer benefits to same-sex partners.

Cook’s disclosure could put at risk Apple’s image in some countries that oppose homosexuality. Many countries in the Middle East and Africa outlaw homosexual relations. In Iran, where Apple was recently seeking permission to sell iPhones, homosexuality is punishable by the death penalty.

But the same social forces at work in America are also prompting the end of discrimination across the world. Same sex unions are legal in Brazil and China decriminalized homosexuality in 1997.

The first gay Fortune 500 CEO

“This is an important next chapter in the business world,” Charles Joughin, national press secretary for HRC, said about Cook's announcement. Despite all the progress, 53% of LGBT people hide their identity in the workplace, and almost 10% said they had left a job because they were made to feel unwelcome because of their sexual orientation, according to the group’s surveys.

Cook said many people at Apple had known that he was gay for years (and in fact, his sexual orientation was an open secret in corporate and media arenas). Being gay has helped him understand the plight of other minorities, he said. "It’s also given me the skin of a rhinoceros, which comes in handy when you’re the CEO of Apple," he added.

Although no other current CEO of a major corporation has come out, former BP CEO John Browne resigned in 2007 just before he was outed by a British tabloid. "I wish I had been brave enough to come out earlier during my tenure as chief executive of BP," Browne wrote in his 2014 book, "The Glass Closet." "I regret it to this day. I know that if I had done so, I would have made more of an impact for other gay men and women."

Browne has since returned to BP to help improve the situation for LGBT employees, notes economist Sylvia Ann Hewlett, CEO of the Center for Talent Innovation. Hewlett's studies have found making employees feel comfortable enough to come out of the closet helps improve productivity, retention and earnings. Cook's announcement should "stiffen the resolve" of other CEOs to make cultural changes for the better, she says.

"Apple could be a thought leader not just in this country but in the world," Hewlett says. "It is a rare opportunity for the private sector to be on the right side of history."

Cook’s essay quickly reverberated across corporate America. Microsoft (MSFT) CEO Satya Nadella said he was “inspired” by the writing. T-Mobile US (TMUS) CEO John Legere simply offered Cook a “right on” on Twitter.

But many states still have laws discriminating against people on the basis of their sexual orientation. Only 21 states and the District of Columbia explicitly prohibit discrimination on the basis of sexual orientation. Alabama, where Cook was raised, allows employers to fire people on that basis, for example.