10 Ridiculous Movie Premises Everybody Fell For
6. Marty Remains Exactly The Same After Changing The Past - Back To The Future (1985)
Rather than worry about paradoxes and all that horrible nonsense, let's look at a simple plot mechanic. Marty has, at the end of the narrative, managed to get his parents together in 1955, albeit in a different way than originally happened. He's also taught his own dad the value of assertiveness and a good left hook, which help with his self-confidence, which gives him and Lorraine the kind of relationship that ensures a stable family life. When Marty returns to the present again, he wakes up to find that George and Lorraine are a successful, happy couple and really good parents, and his own older brother and sister are, likewise, far happier, more successful and better adjusted.
So why isn't Marty himself different?
The answer is that he should be. He would have been raised differently, just as Dave and Linda were: in a strong and emotionally stable family environment, and by a dad that wasn't a total loser. That's how time travel works in the first movie - if you change the past, the ripple effect changes the present. It's only the rubbish sequels that tried to set up alternate timelines requiring charts to keep straight in your head. If it comes to that, all three McFly kids should look pretty different too and maybe even be different ages, as its likely that different zygotes would have been formed by different fertilisation events: it's very possible that George and Lorraine wouldn't have conceived at exactly the same times or even in the same years as they did before. It would have been hilarious if, when Marty changed history to get them together again, he suddenly and inexplicably looked like Eric Stoltz.