When I ask a guy for something, I may get Yes as an answer half the time and No half the time. Yes mostly means Yes and No means No. If the answer is "Let me think about it", that means usually that within 24 hours or so I will get a definitve Yes or No answer.
If I ask a woman for something, I rarely ever get a No. I may get Yes half the time and "Let me think about it" the other half. And moreover, Yes need not necessarily mean Yes, and "Let me think about it" ALWAYS means No - as in: I never hear about it again from that person.
On the surface, that sounds like dishonesty and playing games, and sure is inconvenient not to know what the real answer is. But I am aware of the deeper psychological reasons for not being able to say No to anyone, as I was once like that (and learned through persistence and hard work not to be). It is a matter of politeness mixed with a dose of fear (of being ostracized or something).
And it is certainly much more ingrained in - or inculturated into - women than men. How? Check this post and the 85 comments in the thread under it.
There are two classes of people here, those who will answer reliably and those who will not. Once you know who's who, you should know enough not to ask anything of the wrong class.
If any of them asks you why you never ask them anything, tell the truth: because you wouldn't trust their answer.
This allows the troublesome people the feedback they need to change their behavior, if they wish to, and it will let the deceptive people in peace.
If they accuse you of lacking tact, ask them if that's a euphemism for deviousness.
I've worked 20 years in engineering, and have found this isn't a gender thing, it's a strategy thing. Those who will be honest will be honest; the others won't.
Such a policy will help build your reputation as being honest and forthright.
Cultural training has something to do with it, but who would advocate deliberate deception as a strategy for making friends with people who need honesty from you? This may work in politics, but it fails in engineering and science.
I'm one of those women who had to learn to say no. I never said yes when I meant no; but I did obligate myself against my own self-interest a few times before I got heartily sick of it. It helped that I was an engineer, and had lots of both male and female examples of people who could say no without the sky falling.
Most of the women I know who really can't say no are good at being victims, and get some psychological value out of obligating themselves to others.
As to "let me think about it", I use that response fairly often, but I mean it. Because some people do use it as a deceptive way to say no, I generally add something specific about "If you don't hear from me by Friday, assume I've forgotten and remind me."
Don't forget that this is all very connected to culture as well. I live in Japan, where the mode of discourse is very different. If someone uses an expression like "difficult", it means "can't be done, period.". "I will give it serious thought" means "No. Go away.". And so on and so forth. And you don't even have to go to a different language; British discourse is palpably (and confusingly) different from American, for instance.
But the rub is of course that neither Japanese, not Brits are equivocal. A "I will give it serious thought" is not one bit less clear than "no". If you misunderstand it, it's because you haven't actually mastered the language yet, and is still trying to map words and expressions in one language directly to those of another.
The problem with male/female language - or languages of different socioeconomic classes or whatever - is that you use the same base. It becomes easy to forget that you're actually using different languages.
If I'd do the same experiment (or learning experience) I would ask my students to record not only when they said "yes" when they meant "no," but also when they agreed to a request when they really didn't want to.
Being female often means that you feel guilty saying "I just don't want to do that." Which is just plain wrong.
I've just (at my advanced* age!) managed to get myself into deep doo-doo with respect to an elder (!!!) and senior male colleague who took (what I saw as) my politeness and my welcoming the (as I saw it) interest of a mentor, as personal (sexual) interest.
It's caused me to want to warn all my female students. But, I hardly know what to say that would have reached me when I was 20 (and thought that FOR SURE when I was 21 they would take me seriously!)
Oy, what to do.
*over 18. Heck, over 40. When will I ever learn? And. more importantly, why am I still blaming myself?