WHAT KIND OF RAT MOTHER ARE YOU?
Check out the University of Utah's exercise where you can lick rat pups and see what impact it has on the babies:
"Lick a Rat Pup"
From the website: "Investigate the difference between high and low nutured rats."

A RadioLab episode that touched on this research sums up part of the research:  "...if a rat pup gets a lot of licks, the epigenetic changes cause the pup to grow up and lick its own kids a lot; if a pup doesn't get a lot of licks, the chemical changes cause that pup to be a low-licker. In both cases, the pattern of maternal behavior is carried across generations -- from grandmothers to mothers to grandkids."

So just how do you handle baby rats so that you mimic the rat mother's licks? It's not an easy question, but it's so important.  Match a mother rat,  and you can encourage bonding between rat and human. Too harsh, or off even just a little bit, and you may cause the infants or babies to feel discomfort, to want to pull away, and even to develop negative feelings about what's touching them. Not a good outcome for human-rat bonding.

I don't have the answers about what constitutes "good licking" of infants, or of babies, for that matter, but when I find what I think are good sources and content for what exactly humans should do via touching, or licking, infant or baby rats, I'll post that content here.

I found three videos from the same person which I think are extremely interesting.

Starting with the softest, sweetest, gentlest of touches to infant rats:

Second, these infants seem just a little older to me. I can imagine a rat mom climbing over her babies the way this human is touching this litter of infants. In addition, resting on the human and being touched introduces the human smell. Their encounter with this human is a good one all around:

This third video from the same person is not about touching baby rats, but it is of her touching one of her adult rats, who is a total love bug on her lap, bruxing away. The fact that she can show us friendly, lovely, human-oriented, adult rats, says something very positive about her socialization techniques with infant and baby rats:

And here's another example of touching baby rats from a completely different person: