Lying Online

Here we have a top Prof laying into some virtual world research. His objection is basically, that he can lie about his personal details and generally not take the research seriously. While it is more difficult to lie in a real world laboratory about age and sex, as the experimenter can immediately see through the lies, it is still entirely possible to make stuff up. Even with age and sex, if I fill in the form and submit it with 10 other people, it's possible the experimenter won't realise if I say I'm an 80 year old granny.
An experimental economist discussing the possibility of running experiments in virtual worlds, commented that when subjects show up to his real life lab, they see him looking stern, dressed in a suit and generally being threatening. This, he said, causes them to 'straighten up' and take the whole thing seriously. It's true that in our own in world experiments, we've had people clowning around before (sometimes during) and after the work. We just accepted that as part of the world's culture, although we did take steps to try to get people to take the work seriously (mainly by explaining what we were trying to do, rather than using visual cues like the experimental economist, although we did wear suits and our lab was as 'serious-looking' as we could make it).
So here's the question - how can virtual world participants be encouraged to take online research seriously, or should no attempt be made to change their online behaviour?