So in recent news, apparently feminists discovered, that apparently lot (all?) drug testing is done only on MALE rats. This supposedly is since "female hormone cycles add extra variability, which testers would rather avoid so we can get more confidence with fewer rats." So then this created a furore ("anti-woman conspiracy to avoid detecting sex-specific drug effects") which President Obama now has addressed in some executive order than they should use co-ed lab rats for testing from now on. Presumably, tests will now cost more, but now will detect gender differences (if any). I personally had had no idea of the male rat thing at all, until just heard about it in the news. Which leads me to wonder: what the hell do they do with all the female rats? Also -- which none of the news media I saw mentioned at all -- there is the fact that only females get pregnant. So if you wanted to find teratogen effects on embryos, presumably you'd need to test on pregnant females. But they just aren't mentioning that. Anyhow. So now I'd like us math-funners to actually take a look at the advantages and disadvantages of female rats, without the baloney. I suspect that: for some kinds of tests, you'd be better off using 100% females, for some better off with 100% males, and for some better of with co-ed, but even then the optimal sex ratio would probably not be 50-50. Be good to figure this out. Apparently there is big money involved. Females have two X chromosomes, and in each cell, one of the two (randomly selected) "deactivates" so they effectively only have 1 per cell just like males. (Otherwise, they'd be in trouble, would be like XXY males, mental retardation, gonad problems, greater cancer and heart issues, sterility, and stuff. Although apparently a goodly percentage of XXY males do not experience problems, perhaps because they can deactivate one of their X's? -- overall, it is bad for you.) As a result, which is well known to biologists but not too well known among non-biologists, females are effectively "chimeras" -- mixtures of *two* individuals. As a result, women are believed to have greater smell-abilities than men, plus a woman carrying a colorblindness gene on one of her X's can effectively have FOUR-color vision, which no man has (a man with the gene is just colorblind, i.e. 2-color vision). Anyhow, as a result of this, it seems to me if you test a drug on a female rats, in some ways you are getting two tests for the price of one. For example if you wanted to know: "does this drug cause cancer (with the mechanism related to something that is present on some X chromosomes) it seems to me you'd be better of testing it on 100%, or mostly, female rats. Meanwhile, I buy the whole hormone argument. If you felt confident enough a priori that some drug had nothing to do with hormones and gender, and if it is true hormonal cycles cause extra noise, then you presumably WOULD be better off testing it on male rats only. If your confidence of that was not 100%, though, then you'd want to test it on females too, but probably the optimum strategy (yielding the greatest utility per rat, for some definition of "utility") would be some sex-ratio between 50:50 and 0:100.