Remember kids... always wear a helmet. (The almighty bikeradar helmet thread)

In a similar vain as others have mentioned what if the injury sustained during a is not a head injury but a bad hand laceration. Gloves are recommended cycle wear, should the victim receive less compensation for their injury because they were not wearing gloves.

I think bike helmets are more strongly recommended (mandatory in many countries) than gloves. But the fact of the matter is you'd struggle to prove that a helmet would have prevented injuries anyway.

Do you reduce the amount of compensation a rape victim receives because they walked home through a unlit foot path because that is not recommended?

I would say that people have an inherent right to walk through unlit footpaths, should they wish. This should not effect how they are viewed in the eyes of the law. Not really sure the same should be said for the use of bike helmets though.