No, and that's what I was trying to say in the previous post. Looking at helmeted and non-helemted people doesn't work when you want to see if putting on a helmet makes people take more risks. Many of the riders who wear a helmet regularly are law-abiding, considered, uptight middle-class people, whatever you want to call it. Wearing a helmet is often one aspect of a generally responsible and safe behaviour, so it often goes hand in hand with safe cycling. Also, the fact that roadracers are forced to wear them at races and thus use them during training may do a number on the stats, too. They are experienced, safe riders who rarely crash (in training, anyway), so they bring down the average.
Of course, this doesn't mean that if somebody puts on a helmet for whatever reason, (s)he becomes a more experienced, responsible, safer rider instantly. Not at all.

Looking at deifferent people doens't tell you anything about what will happen If Joe starts wearing a helmet. You have to look at Joe (or the general population) before and after he started wearing one. You have to do a diachronic analisys instead of a synchronic one to get any meaningful result.
Which is where this comes in:

"Helmet laws in Australia, New Zealand and parts of Canada [15] have resulted in the great majority of cyclists wearing helmets, but there has been no reduction in rates of head injury relative to cycle use. An analysis of enforced laws in these countries found no clear evidence of benefit [16].

Casualty trends from other countries where helmet use has become significant also show no reductions in serious or fatal injuries attributable to helmets. In the USA, an increase in helmet use from 18% to 50% of cyclists over a decade was accompanied by a 10% increase in head injuries. There was no clear evidence of any increase in cycle use, which may have declined.

More localised studies have also failed to find population-level evidence of a significant benefit from helmet use."

Risk compesation has been shown to exist, too: "Another possibility concerns so-called 'risk compensation' - the tendency or willingness of people to take greater risks when they feel better protected. There is clear evidence of this, particularly amongst children, and it is quite likely to be a subconscious reaction. If people take greater risks (such as riding in places requiring a higher level of skill) due to a misplaced belief that their helmet makes them safer, they could be more likely to experience a crash."


But here we are probably both getting out of familiar territory... I was sort of hoping that we would go into interesting topics like this one: http://www.cyclehelmets.org/1081.html