I just read an interesting article on CNN. Marital strife around the time an infant is 9 months old can affect his or her ability to stay asleep through the night at 18 months of age. You can click here to read the article.
It makes sense that even at a young age infants can pick up on stress. I just find it interesting that it can affect one's ability to sleep through the night later on. I suppose adults have a hard time sleeping through the night when they are stressed so the same would hold true for little folks.
Hm. Something to think about when applying the whole sleep training concept...