How can I develop a thesis statement from a factual statement? I need to rewrite the following factual statement to make it an effective thesis statement: Many women earn less money than men do, in part because they drop out of the workforce during their child-rearing years.

Whether we like it or not, when it comes to academic writing, everything is an argument. A strong essay does not simply describe a topic; it takes a position and defends it, from the lead-in through the concluding paragraph. A thesis statement lets the reader know what the essay intends to argue, and it prepares them to look out for supporting evidence. A strong thesis should answer the following questions: what is the topic of this essay, and what is the writer’s argument?

A factual statement is a great place to begin. The way you rewrite it will depend upon the argument you intend to make. Let’s look at a few options using the following prompt: “Many women earn less money than men do, in part because they drop out of the workforce during their child-rearing years.”

The average American woman earns 19.5% less than her male coworkers, in part because women drop out of the workforce during their child-rearing years.
This thesis does not improve much upon the original statement, but the introduction of pay-gap data makes the concept of earnings more specific. But what about the second clause, "in part because women drop out of the workforce during their child-rearing years"? Among other problems, this sounds like all women "drop out" as soon as they're ready to have children. This could also benefit from some statistics.
For now, let’s try to strengthen the argument. We know what the problem is, but what can be done?

In order to close the 19.5% pay gap between American women and their male coworkers, employers should be required to provide daycare benefits so that fewer women will be forced to interrupt their careers, and earning potential, during child-rearing years.
This thesis suggests that there is a causal link between women leaving to take care of their children and women earning less and that the problem can be solved with access to affordable childcare.

In order to close the 19.5% pay gap between American women and their male coworkers, employers should be required to provide fertility benefits so that fewer women will be forced to interrupt their careers, and earning potential, during traditional child-rearing years.
Many companies are starting to offer benefits that allow female employees to freeze their eggs or undergo IVF treatments so they can worry less about their “child-rearing years” and more about their work. This thesis suggests that delayed pregnancy could close the pay gap.

While some portion of the gender wage gap may be attributed to women dropping out of the workforce during their child-rearing years, employers should not attempt to increase retention rates by offering fertility benefits to female employees. Instead of social engineering, they should simply pay women more.
Some topics are so divisive that fair arguments are bound to be made from multiple perspectives. Your thesis should clearly illustrate the position you choose. This one suggests that delayed pregnancy initiatives are manipulative and that the best way to close the pay gap is to use that money to pay women more.

Because many women drop out of the workforce during their child-rearing years, they do not reach managerial positions at the same rate as men, which contributes to the 19.5% gender pay gap. Employers should increase their retention of women by developing mentorship programs with female executives.
This thesis argues that the pay gap exists because women are not around long enough to be promoted to higher-earning positions. Data actually suggest women earn ~20% less doing the same exact job as men. While this is still a perfectly serviceable thesis, it demonstrates the way a “factual statement” can be used to support either side of an argument, depending on how the writer frames their position (and what they leave out).