1. Long ago, the newspaper reported about a study of single unskilled women in south Chicago, where it turned out that the women with kids tended to work more hours for higher wages, and often commuted further to their jobs.
2. Another study, reporting that a disproportionate number of people change jobs in pursuit of higher pay after the birth of their first child. I was one of those people, and I know a lot of other people who did the same.
3. Completely anecdotal and informal survey of colleagues, that the people who went into the thankless but higher paying jobs in project or middle management are exclusively parents. I've never met a project manager who was single.
All of this is with the usual caveats about "studies" and anecdotes, but the suggestion is that economic motivation can make people more productive. Or at least, it's enough of a counter-argument to the conventional wisdom to justify reserving judgement in the absence of more robust evidence.