What psychological phenomenon do you believe in but cannot prove? I'd have to go with birth order. Having grown up with three siblings, I can't help but be convinced that my birth order (I'm the second oldest) has had a profoundly important influence on my personality.

That said, birth order is mostly bunk. Numerous scientific studies and meta-analyses have found that the phenomenon doesn't seem to exist. There are few, if any, personality traits that consistently correlate...