Learned liking versus inborn delight: can sweetness give sensual pleasure or is it just motivating?

In this study, we separated for the first time the learned liking for a particular level of sweetness in a familiar drink from the infantile delight in sweetness as such ("the sweeter, the better"). It is widely assumed that sensing a liked food or drink evokes a pleasurable experience, but the only psychological evidence for this assumption has been tongue movements that are elicited specifically by sweet taste in animals and human neonates. We found that adults felt such movements in response to drinking juice at both their personally preferred level of sweetness and levels they deemed so sweet as to be undrinkable. Yet only the intolerably strong level of sweetness elicited enjoyment of the experienced movements, elevation of mood, and a sense of smiling. Hence, the pleasure that adults experience during ingestion could be exclusively linked with the congenital sweetness reflex that sends mother's milk down an infant's throat.