I really don’t think Serena Williams needs any sympathy from me, particularly when it comes to talking about earnings, but it still bothers me a bit that she is frequently referred to as one of the ‘highest earning female athletes ever’, rather than just ‘highest earning athletes ever’. Then again, it is not quite that simple.

There are two things to get out of the way before looking at the earnings gap in women’s sports.

The first is that I am obviously against paying women less than men for doing the same jobs. Of course I am. If I found out that I was in that position and that a company was deliberately paying me less than a male colleague, you bet I’d make a big fuss, lawyers and all. But it is not quite that simple.

My real framework in judging pay comes from being an economist, and that means accepting that the market system works (yes, I’m that kind of economist). If I was being paid less, it might mean that I had accepted less and a male colleague had refused to accept dothat. (And don’t even get me started on the whole women-make-72-cents-for-every-dollar-a-man-makes thing. Those stats are usually incredibly poorly constructed, and frequently compare women working part time to men working full time. But that’s for another blog post).

So back to Serena.

Forget the extra $3.9 million she picked up for coming out on top at the U.S. Open last week: if you look at her winnings to the year ending in June, she made a cool $11 million, which as it happens is a lot more than say Tiger Woods, who amassed a paltry $6.2 million over the same period. Forbes magazine puts her on the top 100 list of top earning athletes – only one of three women to land there.

As this article from the Washington Post details, Serena benefits from the fact that as a female player in tennis she amasses exactly the same Grand Slam earning as her male colleagues. The Women’s Tennis Association (WTA)  is proud that this has happened, and of course they should be. Compared to other sports, there is hardly a gender gap at all between male and female earnings, as The Atlantic neatly details in this analysis.

But is it fair that there is a gender gap at all in sports? Shouldn’t female golfers be as rich as male ones? And why is Serena Williams one of only three female athletes on that Forbes list of the top 100 highest paid athletes (a tally that includes endorsements as well as winnings). That is pretty sad really: the percentage of top earnings athletes that are female is apparently little more than a rounding error.

I have looked around for someone to blame for this, but the truth is that it comes back to the free market, and the fact that women’s sports simply do not attract the same number of viewers that male sports do. In soccer, the Women’s World Cup exists, but it is not ‘THE World Cup’. Women play basketball and golf and hockey, but the same number of people do not watch as is the case for the male versions. That might change over time, but clearly it is not close to changing yet. It is not ‘fair’ but only in the same way that a mediocre hockey player can command a pittance compared to what a star can negotiate.

So how did Serena Williams claw her way on to that top 100 list anyway? And why does women’s tennis pay as much as men’s? To me, it is because as a sport, it does bring in the eyeballs to the television sets, and because they have watched that has allowed plenty of people to realize that Serena Williams is indeed one of the greatest athletes of her generation, male or female.

It all starts with drawing the crowds, and women’s tennis has figured out how to do that in a way that other women’s sports have yet to do. As WTA head Stacey Allaster says, “We are fundamentally in the entertainment business”.

Let’s figure out how to get women’s sports perceived as more entertaining, and then hopefully the money will follow.