Contributory negligence — apportionment

A man is a front-seat passenger in a car driven by a friend. The friend drives negligently and collides with another vehicle. The man was not wearing his seatbelt. Medical evidence establishes that had he worn the seatbelt his injuries would have been substantially less severe, though he would still have suffered some injury from the impact. The man sues his friend in negligence. Liability for the collision is admitted. The defendant argues that the man's damages should be reduced because of his failure to wear a seatbelt.