There’s been plenty of research to prove that diversity in the workplace makes business sense: McKinsey, for example, has found that gender-diverse companies outperform others financially by 15%; ethnically diverse ones do so by as much as 35%.

And yet across many industries, there is still a big gap between the number of men and women in the workplace – particularly further up the leadership ladder. What’s going on?

Theories abound. For Sheryl Sandberg, Facebook’s chief operating officer and author of Lean In: Women, Work and the Will to Lead, women hold themselves back in different ways, jeopardizing their prospects for promotion. Anne-Marie Slaughter points to workplace cultures, which have yet to adapt to the dual role many women - and men - play as both workers and caregivers.

As part of a new report, The Future of Jobs, the Forum spoke with hundreds of chief human resource officers from more than 350 leading companies to find out what’s really to blame for the dearth of female leaders. They pointed to several causes.

Across all industries, almost half of respondents – 44% – said that both unconscious bias among managers and a lack of work-life balance were significant barriers to gender diversity in the workplace. Almost as many – 39% – pointed to a lack of female role models. Although women now outnumber men at university, and graduate in higher numbers, 36% of respondents still said there weren’t enough qualified women for the positions they’re looking to fill. Only 6% blamed a lack of parental leave, and 10% said there were no barriers.

While the report’s findings might be frustrating for those pushing for gender equality at work, there is some good news: most respondents predict that the more gender balanced composition of today’s junior roles will be reflected in mid-level roles by 2020, and that the gender breakdown of today’s mid-level roles will similarly carry through to senior roles.

The Annual Meeting is taking place in Davos from 20 to 23 January, under the theme “Mastering the Fourth Industrial Revolution”.