The unfortunate condition of looking sort of like a caveman, even if they're not that ugly/don't have that big a nose. Affects girls ranging from ugly to mildly pretty. Main cause: Hair that is long, parted down the middle, and/or wavy. Somehow, you see a girl like this and think "Neanderthal" even though she's not ugly.
*Whoah, that girl looks like a caveman! But she's not ugly...Oh, it's the hair. She has Neanderthal syndrome.*
If you or someone you know suffers from Neanderthal syndrome, a side part is strongly recommended. Many lives (or reputations) could be saved by such simple measures.