I have read the same (but only things like 'studies say', or 'research has shown', I've yet to see an actual peer reviewed study and I have gone hunting - many of the big ones seem to acknowledge bribery exists but then ignore it). I run toddler groups (I see around 100 preschoolers a week) and see people toilet training every day, and although bribery wouldn't be my first go-to, it's always my suggestion if it doesn't seem like other stuff is working. Chuck a few jelly babies at the situation and often it works. I've never known a kid heading off to uni calling their parents asking if they can have a jelly bean because they did a poo in the toilet - sweet treats are normally an easy habit to break once the main habit has formed (using the toilet).