If there is such assumption, it would be either correct or incorrect. If it is incorrect, there would be at least some businesses trying to challenge this assumption - it's pretty hard to imagine why in hundreds of years of doing business and pretty much every assumption in there about doing business being challenged nobody would ever think about trying to test this particular one. There are lot of businesses that would give everything for 18% chance over the competition - and among all crazy ideas some would definitely try this one and say "why don't we just hire only women and get 18% edge?" Once they did it, they'd discover they get the same output for 18% cheaper and would have considerable success. Heck, they'd go as far as pay 8% more than competitors to attract only the best and still have 10% edge! Once they had it somebody would think to ask how they did it and discover they did it by challenging this particular assumption and finding it wrong. So the assumption eventually would be widely known to be if not totally wrong than at least questionable and having very strong evidence against it. Has it ever happened? If not, then why not?
Yes, the market is so huge someone would have tried this approach. You would at least expect a few case studies showing increased cost efficiency in mostly women work forces. That none exist, is pretty damning.
