The single cause

"It's like a case study of twins but instead it's a twin study of one."
"A onesie?"
"Sure."
"And we can change one thing about you and see how it plays throughout your lifespan."
"Like making my hair blond?"
"Sure.  Or red.  Whatever you like."
"How can you tell what's different?"
"We take first person video in both trials and then the computer scans for differences between the two sequences.  Any differences it finds, it keeps."
"And typically how much difference do you find?"
"What we typically find is that once the two trials start to diverge, the split widens quickly until they are completely divergent."
"Everything is different?"
"Eventually, yes."
"How long does that take?"
"It depends on how soon the difference affects someone's interaction with the test subject."
"And will I remember any of this?"
"Traces.  But not much, no.  You'll just have a video copy."
"And of my real life too?"
"If you want it, yes.  That costs extra."