''The Man Who Folded Himself'' is a 1973 science fiction novel by DavidGerrold that deals with time travel. It was nominated for the Nebula Award for Best Novel in 1973 and the Hugo Award for Best Novel in 1974. Pretty clearly Gerrold's attempt at writing the end-all and be-all time-travel looping novel, it apparently had another [[ScrewYourself f-word]] in its title, but the publisher wouldn't go for it.
The story follows Daniel (in 1975), a college student, whose uncle increases his allowance (to help with living expenses) if he promises to keep a journal. After his uncle dies, Daniel inherits a "Timebelt" that allows him to travel in time so he goes into the future, meets his future self and makes a large winning on the horse races. Soon after he realises he has to accompany his younger self to the previous day at the horse races and make the same results.
Over the course of the story he learns more and more about the Timebelt and ponders how else he can help himself through the time travel...
!!This novel provides examples of:
* AlternateHistory: Dan changes in time cause a few of these, though he "excises" them back afterward for being too different from the culture he is familiar with. The most radical change, and the quickest to set back, is the timeline where Christianity never came to be. At one point he stops himself from preventing the assassination of [=RFK=] after President Kennedy's efforts accidentally lead to a Coca Cola bottle being discovered in the ruins of Pompeii, thus providing public proof of time travel being possible -- as opposed to ''going back to pick up the bottle''.
* TheFifties: Daniel goes back to live through this entire era a few times, as he feels it's the best and most hopeful point in American history.
* FutureMeScaresMe: Those hollow-eyed old guys having orgies at their shared mansion creep Daniel out.
* HitlersTimeTravelExemptionAct: Subverted. First he kidnaps Hitler as a baby and drops him off into the future. This does not turn out so well, and so he "excises" doing this, but then ensures Hitler is assassinated by his generals in 1939.
* MindScrew: The entire ramifications of this particular form of time travel are hard to wrap your head around. Also, see Temporal Paradox below.
* [[spoiler: MyOwnGrandpa See Temporal Paradox]]
* PopulationControl: Daniel speculates at one point that the Timebelt might have been originally invented as a humane way to dispose of excess humanity, as anyone who uses it [[spoiler:never reappears in their 'original' timeline but in a splinter universe]]. Thus thrill-seekers, sociopaths, or criminals become somebody else's problem.
* ScrewYourself: A ''lot''.
* TemporalParadox: Mostly averted. [[spoiler:Rather than simply traveling through a single timeline, history splits off wherever Daniel travels. Whatever moment in history he "leaves" keeps going on forever, with Daniel simply having vanished.]]
** In one of the most oddest cases of an ontological paradox [[spoiler:Dan meets Diana who is an alternate version of himself who had a sex-change while in the womb; or quite possibly Dan is a male version of Diana. Either way, Dan becomes his own mother and father as a result and Dan raises himself as his uncle Jim. Confused?]]
*** When Dan and Diana first have a baby together, it's a boy (who grows up to be Dan - see '''Time Paradox'''). But Diana is so upset that she didn't get the girl she wanted, she goes into the future and gets medication to force the gender of the baby, then goes back and gives it to herself just before she has with Dan. Thus a new timeline is created where they have a girl (who grows up to be Diana). Dan is then upset that they didn't have a boy, and pulls the same gambit, supposedly budding off yet another timeline with a boy. And so on ad infinitum....
* TimeTravelForFunAndProfit: Daniel first uses his belt to cheat at horse racing. This would have bitten him in the ass, as winning too often draws the wrong sort of attention, but he is forewarned by his future self. After that, Daniel gets a bit more canny (and wanton) about abusing time travel for fun and profit.