We know that when a large corporation has more women on its board of directors, the company is likely to be more innovative. We know that companies who transition from all-male boards to those that are 30 percent female see their net revenue margins increase by 30 percent, on average. We even know that, if there is even one woman on a company's board of directors, attendance is better at board meetings.

Yet somehow, in the U.S., large public companies can't seem to seem to make any progress at all in getting more women onto their boards of directors. A new report by executive search firm Egon Zehnder finds that among large publicly-traded companies in the U.S., just 20 percent of board seats are held by women. That compares to 21 percent two years ago.

Egon Zehnder studied the board composition of public companies in 44 countries. Globally, they found women now hold 18.5 percent of board seats, compared to 13.6 in 2013. In Western Europe, progress has been more impressive, with women holding 26.2 percent of board seats, up from 15.6 percent.

A recent survey by theBoardlist, a group that seeks out women qualified for board seats and helps connect them to search committees, asked why the representation of women on boards was so low. The respondents, who were all women that are looking for board seats, overwhelmingly said the problem was that search committees didn't look outside their networks, and that those networks do not generally include women. Gender bias, often cited as a cause for women's lack of participation on boards, was a distant second.

Egon Zehnder attributes the U.S. lack of progress to somewhat different factors. "While U.S. businesses prioritized engaging a female workforce, contributing factors such as maternity benefits and childcare were largely overlooked," the report says. "This lack of benefits has hindered women from looking beyond the childbearing years to reach the executive positions that enable them to enter the pipeline for board-level positions. "

These Countries Are Doing Way Better Than the U.S.

Many of the European countries with the highest level of gender parity on their boards, such as France (38 percent of French board seats are held by women), Germany, and Italy, have managed to do so at least partly because they've recently been subject to quotas. Other countries, such as the U.K., have gender diversity champions who have made excellent use of the bully pulpit, notably Lord Mervyn Davies, who is calling for 33 percent board participation by women by 2020. (The U.K. currently stands at 26 percent)

But one need look no farther than Canada, which does not have quotas for women on boards, to see that progress is possible. Between 2014 and 2016, the percentage of women on the boards of the largest publicly-traded Canadian companies increased from 18 to 25 percent. In the same time period, in the U.S., the percentage of women on boards actually fell, from 21 to 20 percent.

In Canada, the report notes, unlike in the U.S., there have been strong efforts to emphasize diversity at the highest levels of government and society, most notably that of Prime Minister Justin Trudeau, who appointed 15 men and 15 women to cabinet positions. The report quotes Trudeau, speaking before the U.N. General Assembly in September: "In Canada, we see diversity as a source of strength, not a weakness; our country is strong not despite our differences, but because of them."