This is interesting to me. While I can agree that at 18 or 20 months a toddler may simply be saying what they wish to be true, I feel children can "lie" with the intention of deceiving someone else before age 6-7. My 3 1/2 year old will say things like, "Mommy said it's ok," (when I did not) or when asked if she has a marker say say and hide it behind her back.
While I'd say it's developmentally normal, I am starting to discuss with her that it's not ok. She can't tell Daddy that Mommy said something was ok when I didn't say that. The look on her face pretty much confirms she knows she was caught red-handed too.
