Many companies are buckling down on their diversity efforts, some with better results than others. But most of those initiatives focus on employee bases, with few businesses giving the same attention to diversity at the board level. That’s a problem. Here’s why it persists and what it will take to fix it.

In 2015, 73.2% of new public board appointments went to CEOs and CFOs, according to one industry report. Among last year’s Fortune 500 companies, just 21 (or 4.2%) of CEOs are women, nine are Hispanic, and five are African-American. In fact, in the entire history of the Fortune 500, there have only ever been 15 African-American CEOs.

As long as new board positions go primarily to public-company CEOs and CFOs, women and minorities will continue to be largely excluded.

As long as new board positions go primarily to public-company CEOs and CFOs, women and minorities will continue to be largely excluded. Considering board members with different backgrounds, from academia to government and nonprofit sectors, would expand the slate of qualified women and minority candidates considered for leadership roles by corporate boards.

While age discrimination persists in many companies, we’re seeing the reverse at the board level: The average age for new board members is 58, and among active board members it’s 63, according to the Wall Street Journal. Some 19% of directors are older than 69, but fewer than 1% are under 40. As one Bloomberg editorial puts it, “corporate boards shouldn’t be retirement homes.”

While experience is valuable and retirees often have the bandwidth to devote to directorships, a well-rounded board should also include people well-versed in the latest technologies and trends–knowledge more likely held by younger, currently employed executives. In age and employment status, as with so much else, finding the right balance is key.

There’s a well-documented shortfall in women and underrepresented minorities graduating from engineering schools, pursuing technology-based careers, and breaking through to leadership positions. Limiting (non-CFO and non-CEO) board appointments to candidates with engineering backgrounds is a sure route to continuing that trend.

There are plenty of brilliant business and academic leaders who didn’t major in engineering but are still qualified to steer a tech business. Can you imagine anyone not wanting economics majors Sheryl Sandberg or Meg Whitman, or psychology major Mark Zuckerberg, on their corporate boards?