Back to the Future is one of my favorite movies. It's science fiction, it's funny, it's got decent action and some of my favorite actors. It was also a movie I watched at the right age. All of that said, there has been something that has bothered me about the movie since I first noticed it. That is that Marty McFly comes back to a world in which everyone and everything he knows has been changed.
It's actually pointed out at the end of the movie. His father is a writer, his mother doesn't drink, his siblings are more successful and his girlfriend even looks different. All of this seems great, if you're talking about a five minute scene, but think about it from Marty's point of view. He is living with people who had different memories. His entire life would have played out differently and even if the people who live in his house are more successful and better people than his family they aren't his family.
After watching a recent movie in which time travel was discussed I was thinking about this again and I think I have the solution. The Marty after Back to the Future is a different person as well. It just takes a bit of time.
In the past he accidentally changed time. But as we saw time in the Back to the Future universe doesn't change immediately. Marty begins to fade. Meanwhile newspapers, which have traveled through time, change to match the new reality. So when Marty returns from the past he has a few minutes when he is mostly the same person. That person sees how reality has changed, but then over the next few hours that Marty becomes the Marty who was raised by those people. A version of himself that only seems the same to us because we don't know him all that well. All we've seen of what Marty is like is part of one day and a bunch of very unusual circumstances.
What clues are there? The first I already mentioned, his girlfriend looks like a different person. Something that he has already changed enough to not notice. But there are other clues. Why does Doc Brown show up for Marty at this moment instead of picking him up before? Sure he might be avoiding damaging the timeline, or perhaps none of the events that they are trying to prevent happened before he was changed in the timeline. This would explain why Doc is wiling to change things. It also explains a personality quirk that appears out of no where.
This version of Marty hates people calling him chicken. There is no evidence, (that I recall) of this personality quirk in the first movie. But think about someone who grew up with a father who had learned the importance of courage by saving that person's mother from a dangerous situation. These are two people who would emphasize the importance of not being a chicken to their children. Perhaps to the point where they might do something stupid to prove they aren't a chicken.
Does this fit into the way that time travel is explored in the movies, it depends on how long this effect takes to happen. Sure Marty doesn't change when he goes to the darkest timeline where Biff runs everything, but that could be because there was an invisible clock we didn't know about and in a few hours Marty would have turned into the person raised in that reality.
Whatever is true, I like the idea that at the end of the back to the future movies Marty is living with a family he actually knows and who know him rather than him being permanently displaced in a reality where his family are people he didn't grow up with.