Why do we call our lovers "baby"?

It is common in American English and culture to refer to one's lover or significant other as "baby" or "babe", for example:

Come on baby, light my fire! ¹

or

I got you, I won't let go. I got you to love me so. I got you babe! ²

Where does this usage come from? What's the history behind referring to one's significant other as "babe" or "baby"?

¹ Credit to The Doors

² Credit to Sonny & Cher

There is a kind of a pop evo-psych explanation lurking here. Lovers are called "baby," because they tend to evoke the same kind of feeling one experiences with a baby. Babies are "designed" to do this; they need to excite the same passions as lovers to be taken care of, especially human babies, whose species are huge outliers in the amount of parental (and paternal) investment they require among the mammals.

If one wants to be cold and mechanistic, one might say that babies and lovers exploit the same neurological circuits; the wires are quite literally crossed in the brain. Because evolution most often proceeds by co-option of existing behavioral repertoires, this make sense; natural selection can only search within a small neighborhood of a population's current fitness, i.e. a small neighborhood of its members' current behaviors, within the fitness landscape. When we underwent the transition from our ape-like ancestors, who probably lived multimale-multifemale social groups like their descendants the chimpanzees, and whose sexual competition was similarly intense, to our derived strategy of long-term pair bonding and paternal investment, it is a strong hypothesis that babies must have particularly exploited the emotional circuits that deal with pair-coupling and sexual attractiveness. Here is an in-depth treatment of this hypothesis by a certain David Brin.

Thus, as both babies and lovers evoke similar feelings, it makes sense that the words associated with these feelings would be in some sense interchangeable with one another, though I wouldn't extrapolate that understanding too far.

I'd also venture that "baby" as a pet name is directed much more by men towards women than vice versa, though I have no direct evidence on this point as yet. This is because neotenous features, such as large eyes, fine hair, and high voices, are important in determining women's attractiveness, as their age really matters with respect to their expected lifetime fertility, the ultimate metric that natural selection evaluates. Notice that babies share many of these same attributes, which are lost in puberty in men through the action of the "master" hormone testosterone. Men put much more of a premium on neoteny and physical attractiveness in general, so you would expect a difference in men calling their female lovers "baby" as compared to vice versa across cultures.

Etymonline says it's certainly older than rock 'n' roll.

As a term of endearment for one's lover it is attested perhaps as early as 1839, certainly by 1901; its popularity perhaps boosted by baby vamp "a popular girl," student slang from c.1922.

Although I realise a behavioural explanation might hurt long established anthropocentric viewpoints, I believe the answer to this question is better understood if one adopts an ethological viewpoint.

In ethology one describes tenderness as a way of consolidating mating bonding. It is indeed a well documented pattern in many evolved animal species that individuals in a mating relationship with another individual of the same species (partners) will regularly express tenderness as a means to trigger reciprocal feelings, lower the partner's defence and ultimately enhance the chances of a successful relationship.

When in "non mating mode", the standard relationship between two individuals of a given species is dominated by competition for existing resources (predation territory, food or other survival-critical resource...). This is a situation where the relations between these two individuals can be marked by aggression and defence reflexes (Konrad Lorenz in his book "On Aggression" has shown how aggression was an intra- rather than inter-species phenomenon, essentially because individuals of the same species are in the same niche).

When in "mating mode", instead, mating partners must find a way to form a permanent or temporary relationship based on mutual respect and collaboration driven by common genetic (first) and survival (second) imperatives (see the "Selfish Gene theory" for the priorities).

In many superior primates for instance, the male will engage in gift exchanges or grooming to lower the defence barrier of the partner.

Transposed to human social behaviour, calling your partner "Baby" suggests "I feel like protecting you" (i.e. "I'm not in an aggressive mood, please consider lower your defence reflexes"). If the partner accepts the implicit mating relationship, or consider it an option worth exploring, he or she will find a way to reciprocate the feeling through the emission of a comparable signal, i.e. welcoming the protection.

There are many other ways to express the "I feel like protecting you" message but since, in all species, babies are the very symbol of individuals needing protection (an obviously indispensable feeling if the genes are to be propagated to the next generations), it is one of the most efficient vector to convey this feeling. Konrad Lorenz again has shown (in his book "Studies in Animal and Human Behaviour") how mature individuals are universally responsive to baby features such as high pitch voices, roundish features and facial expressions (changing nappies is too recent in the evolution to effectively provoke that same "Let me do it first feeling" ;-).

It's really just a general term of endearment that some (not most) people use. It's slightly more likely to be restricted to just "the significant other" than Pet, Love, Dear, Honey, etc. Plus Baby seems to have more staying power than most - it's been around a long time, but it doesn't really sound archaic (just twee, to me at least).

Young people in the UK (particularly females) use babes as a singular pronoun when talking to just about anyone (but again, particularly to females).