Riding the greyhound back to school gave me alot of time to think, so this is my hypothesis about reincarnation:
Reincarnation is real, but it doesn't follow an objective timescale. What this means is that when you die you could be reborn as anyone at any time in history or in the future. This would solve the "new souls" problem , or maybe over time there is only one consciousness being born and reborn into everyone who ever lived and ever will live. This means that everyone you meet is a past life or future incarnation of yourself. So anything we do to others we are really doing to ourselves, justifying the golden rule.