The report tells us that 50% to 65% of women at the vice-president level and higher are in staff roles, compared with only 41% to 48% of men, who are more likely to be in the line jobs that lead to the top. This subtler gender gap — one that is difficult to detect when companies only count the total number of men and women at each grade level — is the primary reason we still find so few women in the C-suite (19% in the U.S., according to the same McKinsey report).

To tackle the disproportion, most companies offer mentoring programs, networking among women, and women’s leadership programs focusing on topics like self-promotion and personal branding. These rarely work, and they breed cynicism.

What should they do instead? Follow the 70-20-10 rule.

This obvious solution hit me last year, when I sat on the jury of the Fortune/AON Hewitt’s Best Companies for Leaders. Each contender submitted a thorough dossier documenting his or her company’s approach to leadership development. Invariably, each described the sacrosanct 70-20-10 rule, by which 70% of a manager’s learning and development should come from on-the-job learning through stretch assignments, with only 20% and 10% coming from mentoring and classroom learning, respectively. Following this rule means focusing on the company’s appointment process and succession planning. It requires knowing what are its “pivotal roles,” or those assignments that are deemed essential to an executive’s development because they expose him or her to the company’s primary sources of revenue, strategic markets, or key products. And it requires actively managing high potentials so that they make their way through a logical progression of these key roles. Within this kind of system, mentoring is not a “feel good” exercise; it is a means for identifying and providing what a talented person needs to get into one of the jobs and do well there.

Interestingly, the same companies that followed the 70-20-10 rule described a different approach for tackling diversity in their leadership potential pool. What did they do? You guessed it. They put in mentoring programs, and women’s networks and leadership courses.

All of these have their place in a coherent diversity initiative (Disclaimer: I direct leadership programs for women at INSEAD). But without a foundation in the heart of it all — the jobs and assignments that provide essential experience and exposure — the results will most certainly be lacking.

For our World Economic Forum Corporate Gender Gap Report (PDF), Saadia Zahidi and I asked the top HR person in the largest companies of 20 OECD countries the following question: “Among the assignments that you consider to be business critical/important, what percentage, in your opinion, are currently held by women (e.g., key start-ups, turnarounds, and line roles in key business units or markets)?” The most common answers were “0-10%” or “not measured.”

Women’s assignments must become the number one priority for committed CEOs who want better results. Expecting to get more women into senior management via formal mentoring and self-improvement courses is Einstein’s definition of madness: doing more of the same and expecting a different result.

>> This post originally appeared on HBR.org