Taming rats takes time and effort, and you must put in that time to get the desired effects. If it is a litter of kittens, they should be handled as soon as possible, and every day after that. They will then learn that humans are not a threat, and that they are fun, too! If the rat is older, it is more difficult to teach, as the idea that humans are scary and unknown has already been implanted in its head. If the babies get abused, either intentionally or unintentionally during any time but especially their youth, that impression will stay with them for their whole lives.
Genetics
Behaviour is attained through learning and conditioning a lot more than genetics, but it does play a role. If the rat's parents are flighty or aggressive, the offspring have a higher tendency to become that way, too. Those tendencies can be modified if the kittens are handled frequently, but sometimes some rats are untamable. Behavioural aggression may not show itself until between 6 and 12 months, so a rat that may have been perfectly tame beforehand that suddenly starts to show signs of aggression may be exhibiting genetic aggression.
Environment
Numerous studies have proven that rats function better in large or complex cages. If you have one rat in a tiny cage with no wheel, toys, or anything, it can develop numerous behaviours or mental problems. Boredom is the number one cause of these problems. Sometimes even a rat living with others can exhibit boredom problems. Also, rats are extremely social animals. It is always better to have two than one, so that they will feel safer and happier. Overcrowding can also cause rats to become frightened or aggressive as they feel as if they don't have enough space of their own.
Change In Behaviour
A change in behaviour may indicate that something is wrong. Female rats can become aggressive if they are pregnant or nursing. Male rats can become aggressive if they are housed close to females. If there are too many rats in one cage, they may squabble more often and become more frightened or more aggressive towards other rats or humans. Illnesses and pain can also cause a rat to change its behaviour suddenly. If your rat exhibits signs of aggression or fear that weren't there before, consider having him examined by a veterinarian.
