There's an idea I've seen a number of times that 80% of women have had descendants, but only 40% of men. A little research tracked it back to this, but the speech doesn't have a cite and I haven't found a source.
The reproduction rates for men and women (possibly for the whole history of the species) seems like the sort of thing which could be found out, but I'd like more solid information.
Yup. So what sort of data could either support or make unlikely a 50% cuckoldry rate among ancestral societies? I'm not sure that even a genetic study would give good answers, so we may have to go with indirect evidence.
What's the cuckoldry rate now? Ooh, I found a blog post on it by Razib Kahn (just by googling "cuckoldry," actually). He also links to a paper that finds a bit of a decrease in "nonpaternity rates," and attributes it to widespread birth control (makes sense).
Here is a list of paternity studies. The best study is the modern German study because there was very little selection bias. But there are several surname studies showing <2% nonpaternity rates over centuries. Unfortunately the list lacks citations, though there are some in the comments.