Humour: a double edged sword

In preparing to present a workshop on Ageism recently, I found the need to address the underlying psychology when humour is used to reinforce stereotypes/prejudices. Y'know, the comments that people laugh at as a means to commiserate about their own ageing. So what's wrong with that - you need to be able to laugh at yourself, right?
Humour can be a double-edged sword; it combines the disparaging underlying meaning with the funny part which promotes subtle and acceptable attitudes. This is known as "disparagement humour".
In other words, disparagement humour is paradoxical; it simultaneously communicates two conflicting messages. One is an explicit hostile or prejudiced message, but delivered alongside it is a second implicit message that it doesn’t count as hostility or prejudice because I didn’t mean it —"it’s just a joke.” 
By disguising expressions of prejudice in a cloak of fun and frivolity, disparagement humour appears harmless and trivial. However, a large and growing body of psychology research suggests just the opposite – that disparagement humor can foster discrimination against targeted groups.

Just think about it; would it be acceptable to make racist or sexist jokes in our current society?