I guess it all depends on what you mean by "tardiness."

My eyes aren't good enough to read the small print in this chart. Sad.

That's a very sad comment. You can hardly make a generalization that "parents are, indeed, less available to put in extra effort at work." It's not only false, but offensive. I know for a fact that I've worked just as hard, if not harder, than some of my single co-workers. To pay someone less just because they're a parent is not only discriminatory but in fact NONSENSICAL but it is not based on the person's qualifications and their actual work product, but on an arbitrary judgment about lifestyle. Working parents have enough guilt and stress--working long hours at the expense of their sleep, health and sanity--without people like you passing judgment on them and perpetuating these false stereotypes. It is very sad to me that we life in a culture that tells us we need to be perfect parents, yet doesn't support us in even being good ones.