I could be wrong, but I think the change started happening when they started being more articulated. Or at least this is what I noticed in the Barbie dolls I bought, although those are all articulated. Same thing was going on with the Disney dolls, depending on the years, some of those are a hybrid of non articulated (bendable plastic) legs with the hollow plastic articulated arms. Some of the later Disney dolls were all hollow plastic articulated fully (arms & legs).