Recently there was a program on tv on babies and different "age tests"- for separation anxiety they would put a baby in a room with a stranger and the mother, have the mother leave the room, then wait to see if baby cried or got upset at the mother's absence- it wasn't until age 13months (or so) that the babies got really upset- not being able to understand the concept that mom has not left for good. I wish I could remember the exact age they were, I do know it was older than 12 months because I remember thinking- great, when I go back to work from mat leave my dd is not going to get it that I'm coming back. Maybe the difference was that in the tests the babies were left with another person vs alone? I've wondered this as well as my lo will sometimes wimper when I leave her in her crib and be all smiles when I come running to her rescue.