How are decoys discriminated? The acceleration due to gravity is the same for all. Radar reflectance could be manipulated. Drag and lift perhaps, but can't those be matched with the real thing ? What is it that gives up a decoy as a decoy ?

Yes I can distinguish a falling feather from a falling lead-shot. The more interesting questions is can one not make a featherweight that falls like lead-shot, reflects like lead-shot and be cheaper than lead-shot.

For a warhead decoy ten times lighter it needs to present an area ten times smaller to maintain drag induced deceleration parity. For radio parity the decoy needs to reflect more the actual warhead reflect less. Metal coated kevlar ribbons on the decoy might do it.

What I find interesting is that the curse of rocket equation makes decoys pretty expensive to propel. So if I am forced to incur that cost anyway, might as well put a real warhead on that.