You might think someone must be stupid to sign a contract without even reading it. Yet I see it happen all the time, when contracts are just too long and there's no time to read stuff you suppose you already know. Typical examples are contracts you "accept" online, like TOS's and privacy policies. Nobody reads that. And the funny thing is that the other party usually knows you haven't read it. Websites know that nobody reads their huge TOS's or privacy policies, and it has also been demonstrated (researches, polls, articles). And the salesman telling you "just sign here, it's the usual stuff", they know you haven't read it because they saw you sign it without reading it.
In the case of online contracts, you don't even have a copy of them, so the other party could change the terms and you wouldn't even have a way to know it. Stackexchange could tell me that I agreed to post at least a question per month, or my profile would be deleted. Well, did I agree to that? Maybe, who knows, I don't know what was really written in the TOS when I clicked, and now they might have changed the terms and there could be no trace of the old contract.
So are these contracts valid? What makes a contract valid and binding?
If jurisdiction is important, I'm interested in the EU and the US.