Subject: Why can some people see 3-D pictures and others can't?

I am doing a science project to try and determine why some people 
can see those "Magic Eye" pictures and others can't. 
Some people have told me it is a hereditary trait while others have 
said you just have to train your eyes to see them.
Please help me. I do not have very long to finish this project and I
do not even know where to begin.
Thanks so much,
Beth

Re: Why can some people see 3-D pictures and others can't?