Why do fingernails grow faster than toenails?

This  is not the kind of question whose solution wins Nobel Prizes for scientists or garners prestigious grants for research hospitals, yet the answer is not obvious. The average severed fingernail takes four to six month to grow back to its normal length. The average toenail takes nine to 12 months.

Dermatologist Dr Fred Feldman says that although nobody knows for sure why toenails lag behind fingernails in growth, there are many possible explanations: 

1. Trauma makes nails grow faster.
2. All nails grow faster in the summer than the winter, which suggests that the sun promotes nail growth. Even during the summer, most people cover their toenails with socks and shoes.
3. Circulation is much more sluggish in the feet than in the hands.