Interesting, I'll have to look into this. A quick search was inconclusive but showed a split between "sciency" sources (saying no relation between infant smiles and gas) and "parenty" sources saying it's gas. Thanks!
I meant 'inconclusive' as in "in 2 minutes of searching I found a couple of papers whose abstracts said no causation between gas and smiling, and about four of five 'mummy-type' sites that asserted the correlation." To me that points to 'no causation' being the likely case but I'm not going to just declare it as proven on the basis of the first two papers I turned up.
