Cats, like all mammals, are social creatures. While cats are far less social than dogs, to think of cats as asocial is wrong. The degree of social behavior and the ability of an individual to adapt to a changing social environment varies with species (certainly cats are less social than dogs), breed (Bengal cats are less social than domestic short-hairs), and individual personality. Even individual personality is dependent on factors like genetics (or what we often call temperament) and learning or socialization. So genetics play a big role, at the species, breed, and then individual level.
A recent study looked at how young cats reacted to familiar and unfamiliar people and to a novel object, and showed that the social personality of the father (genetics) as well as the degree of early socialization (learning) influenced the later degree of social stress in human interactions, but only the genetics influenced the (nonsocial) response to a novel object. This is a nice example of the interaction between genetics and learning. And of course, this is true of all mammals.
So cats are less social than dogs… and some cats are genetically at the nonsocial end of the cat continuum. This means that not all cats can be housed socially. This is true of dogs as well, just much less common. But there is that one important variable, learning and the environment, that gives us something that we can manipulate, or a tool to maximize the odds that a cat that we own will be sociable.
The next factor to understand is the concept of critical periods, again common in all mammals. This is another process by which genetics and experience interact: critical periods are genetically-controlled time windows in the brain in which certain experiences are dramatically more likely to be remembered and to influence later behavior. In kittens, the critical period for positive effects of social experience is approximately three to eight weeks of age (with some variability, 2-12 weeks to be sure). Evolutionarily, in the wild, this is the period in which kittens learn who their parents and siblings are: these are “friendlies” with whom you can safely socialize. These are also “not-mates” thus preventing inbreeding issues.
So cats will generally only socialize well with siblings (who they experienced at 3-8 weeks old), and even this is true only in females… males become entirely nonsocial, except for brief periods for mating. In captivity, it is VERY difficult to have more than one male in a household, even if they are castrated, unless they were socialized with a lot of other cats in the 3-8 week window. And remember, this behavior and response is all on a continuum: there are plenty of examples of social male cats but of course, there are many examples of big problems as well.
So given that there is a critical period for socialization at 3-8 weeks, what does this mean for our companion cats? A common issue is a kitten adopted from its litter prior to 12 weeks and who is wonderful with humans (who handled it in the socialization window), but attacks any other cat. Or a young cat that was part of a feral litter, who is socialized only with its littermates and has a very hard time socializing with humans, whom it only encountered well after the socialization critical window, or other cats, with whom it was not socialized and perhaps has even had some negative, competitive interactions.
We have been talking about positive social experiences: how about negative or traumatic experiences? The same effect works in this case: traumatic or unstable social experiences in the 3-8 week window have a much stronger, more lasting effect than the same experiences at other times. A recent study, for instance, showed that poorly socialized cats exhibited higher levels of stress when housed in a social setting, whereas cats with a positive early history of socialization showed no differences in social stress in solitary or social housing situations.
So what does this tell us about introducing a new kitten into a household? First, take into account the history. In many cases, for instance in shelter adoptions, there is very little history, but if there is any information about the personalities of the parents, this may be important. Then, what is known about the kitten’s early socialization? Finally, how was the cat housed, and at what age, in the shelter, or even elsewhere? All of this information will tell you something about how the cat will react to a social situation.
Then, when introducing a new kitten into a feline social setting, the two big rules are: Go Slow and Keep It Positive. Introductions should proceed slowly, only as fast as the participants want. Keep cats separated by doors, windows, furniture, or baby gates, and introduce them to each other slowly. Keep it positive, make everybody want to play the “meet the new cat” game. Provide food rewards or attention to reward good social behavior: make it a party! But don’t push strange cats together: each bad interaction, hiss, or fight provides a negative connotation to the interaction, and enough of these just teach the cats to avoid each other as much as possible. Finally, encourage socialization as early as possible: the more a kitten is handled (gently and without trauma, of course) by as many humans as possible, and the more different cats it encounters (again, in a positive way), the greater the chances of later problem-free social behavior.