list not working

[Zman350x]

    from __future__ import division
    import time, math

    List = [3,4,6]

    MADlist = List
    for i in range(len(MADlist)):
      MADlist[i] -= mean
      MADlist[i] = math.fabs(MADlist[i])
    MADsum = float(sum(MADlist))
    MAD = MADsum/(len(MADlist))

Here's my code.

I have a list called List. I want to make some changes without actually changing List, so I create another list, MADlist, to make changes to while keeping the original list the same. For whatever reason, it makes the same changes to the original list too! Why?