When a cat shows you his belly, it’s usually a sign of trust because he’s putting himself in a vulnerable position. A cat that rolls on his back and presents his tummy is saying that you’re a friend, and that your relationship is cooperative rather than competitive. However, some cats will also roll on their backs when they want to play, as it gets them into a good position to tackle a proffered hand, so it’s risky to assume that this gesture is always an invitation to a belly scratch, particularly with an unknown cat.
A cat may have mixed feelings, wanting to trust but feeling wary, particularly if he was handled roughly in the past. He might invite a tummy rub, but then become fearful and launch a warning attack on your hand. Such attacks are not usually done at full strength. They’re a way of letting you know that the cat wants to trust but has been mistreated in the past and is prepared to defend himself if you take advantage of his vulnerability.
Kittens that were taken away from their mothers too early and not properly socialized are more likely to behave ambivalently, switching from friendly tummy display to attack mode. Poorly socialized cats also don’t understand the appropriate level of force to use when playing, so their attacks may be more forceful than the typical warning swat or grab and release.
Some cats enjoy a brief tummy rub but soon warn the person off because petting becomes irritating due to static electricity. Cats may also attack when their bellies are petted because they have injuries or other medical problems that cause pain.
For more articles on the way cats think and the reasons why they do the things they do, visit the main Cat Psychology, Communication, and Behaviour page. For a full list of cat articles, see the main Cats page.
Reference: Tabor, R. (2005). 100 Ways to Understand Your Cat. Cincinnati, OH: David & Charles.