I think it can, if your body is ready. I found it brought on false labour for me. Had a few hours of light contractions but they stopped when DD1 woke up. When I went into proper labour a week later it was v quick and easy though, so maybe they did something!
No it can't. In all research ever carried out, it has never once shown to have any causative effect on labour starting.
Nothing short of chemical induction can bring on labour. Sex, like any exercise can cause you to suffer Braxton hicks and it can, after a certain point of pregnancy, make you feel quite uncomfortable afterwards.
Some people will have sex and then go into labour 12/24/whatever hours later. Many thousands more will have sex and not go into labour. It's just a coincidence and because it's an old wives' tale, people tend to insert a causal link there where they don't with anything else that they did 12hrs before labour started. So you could eat a stew, sniff some lavender, see a blackbird and have sex. Labour starts 12 hours later (generally because you're heavily pregnant and the baby is ready to be born) and people don't say "it's that fucking blackbird, I swear", they say "oh it must be the sex". It never is. It's just coincidence.
I ate Jaffa cakes the day I went into labour with both of my pregnancies. I'm trying to convince people to use it as an induction method.
I tried every old wives' tale in the book and nothing would bring me into labour (not even being induced!). There is some science behind the sex myth - namely that sperm contains prostaglandins which help ripen the cervix. I can't believe that the amount is anywhere near enough to get things going on its own. I think if your body and baby are already gearing up for labour it may give you enough of a nudge to send you off but that's about it.
DD was born at 08.40. shortly after 9pm the previous evening, her father and I had had sex. I had also drunk a little Carlsberg Special. Not sure which of those caused the baby to arrive... I couldn't eat my meal the evening before, and the midwife had warned me to watch out for that as a sign the baby was coming.