FOR WRITERS (and readers): Why are heads cut off on book covers?

Now, this wasn't something I thought much about, until I received my new book cover (Isn't it beautiful?). I've always assumed that publishers cut off the model's heads so that readers could imagine the faces for themselves (our imaginations are often times much better than reality). But I found out that there is a subconscious marketing technique at work here. Publishers know that they only have seconds to capture the interest of a reader, whether he/she is looking through books in a bookstore or on the internet. So if you glance at an image that is incomplete your brain registers that within seconds...and you're more likely to take a second look. Then you might be tempted to read the cover, the back blurb, and hopefully, purchase the book itself. But those first few seconds are the most crucial, which is why publishers invest a great deal of research in producing an eye-catching cover.