A little late to the party here, but we might want to adjust for the fact that most S&P 500 companies are matured, and are involved in markets/market segments that have reached the point of saturation - this is especially important to note considering we're looking at a significantly larger portfolio of companies from which women-led ones are selected, and therefore, many smaller companies with more room for growth are included. You can argue that they're supposed to have the innovation-based growth to overcome that, but the fact remains that their "cash cow" (high-revenue, low-to-no growth) divisions will likely account for far more stable revenue than any of their frontier/high innovation divisions with high growth, leading to significantly lower overall growth, and thus less equity value and dividend growth as well (and thus, of course, lower overall returns). Might want to consider that most of the new entrants into the Fortune 1000 might also be experiencing quite a bit more of what we call "bubble-driven growth" considering how frothy the markets were for certain periods.
From the graph, we can see higher levels of volatility, and more significant drawdown among the companies you selected from the Fortune 1000, which may be a reflection of their smaller size and lower stability compared to their S&P 500 peers. S&P 500 companies, being the usual "blue chips", are supposed to be stable, boring companies with good dividend payouts and decent (most likely, underperforming vs underdogs or newer companies that are doing well) share price growth, and that's pretty much their selling point.
From an anecdotal perspective, its like comparing Apple to say, Google at the time of its IPO. Going forward, obviously Google had more room to grow, across the same period. I'll admit it's pretty challenging to achieve statistical balance in situations such as these, where the sample size is relatively small, and where you pretty much have to "take what you can get" with regards to data, but it's important not to state conclusions too strongly.
That being said, I'm sure we're onto something with respect to gender balance in company leadership. It's great that someone around here took the time to establish rough correlation (but once again, far from causation) in the context of this issue. =)
Just saying. I was just a little bummed by how conclusive these results were made to sound by a certain article on Quartz. I agree with the above poster, by the way.
P.S. Feel free to call me out on any mistakes I've made, or if the algorithm is more comprehensive than it seems at first glance.