If you are introducing a new rat to old rats, please follow proper quarantine and introduction procedures. Most rats can be introduced without any problem, so long as you do a proper intro.
How to Socialize a Rat/Mice
If you just toss your rats in together, you could very easily end up with a severely injured or even dead rat.
As for human interaction, if bought from a pet store your rats may need a tiny bit of extra patience when socializing.
It’s good to wait a couple of days after bringing your rats home before trying to handle them, as they’ve spent the beginning of their lives being fearful of humans.
A good breeder will have socialized their rats from birth, and while it is still good to wait a couple of days to let them settle into their new home, socializing these rats will be much less of an issue.
Rescues often try to socialize their rats before adopting them out, but often times these rats come from troubled pasts, and may need a little extra help with trusting you.
You can help to socialize them by talking to them each day. Give them some time before trying to hold them. Offer treats by putting it in the cage near them and talking to them with your hand in the cage, but do not go to pet them just yet.
Once they start associating you with treats, and find you to not be so scary, you can try to offer them treats closer and closer (watch their mouths!) and eventually move on to petting them.
If you are nervous, you can take them out with a towel, but make sure you are in a secure area in case they spook and run from you. When you do start picking them up, do it for only a couple seconds at a time, then reward with a treat that they love.
Do this a couple times a day, and then give them their space. Every day increase holding time a bit more. Eventually you should be able to hold them for longer periods of time and they should come to trust you completely.
You can also try forced socialization once you get basic trust mastered. The name sounds mean, but it’s really not.
It involves holding the rat for at least 20 minutes against it’s will. Some people will let the rat crawl in their clothing, avoiding contact with fingers, while still letting the rat get used to your sent.
20 minutes is important because a rat cannot maintain it’s fear of you longer than this, and once it’s adrenaline levels are reduced it will begin to associate you with calm and safety.