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In the last few years a number of additions to the technical literature on relationships between car size or mass and occupant risk of fatality or injury have appeared. This new information is reviewed, synthesized and used as the basis for additional calculations aimed at better identifying causal factors. It is concluded that if a car crashes head-on into a 12,000 kg truck, the car driver is 36% more likely to be killed in a 900 kg car than in an 1,800 kg car solely as a result of differing Newtonian kinematics. Five studies from two countries consistently support that when cars of similar mass crash head-on into each other, driver risk is inversely related to the common car mass. Size is the dominant causative factor in this relationship, and in the higher rollover risk in lighter cars. Mass and size are causal factors in single-car nonrollover crashes. Mass exercises a dominant causal effect on car driver risk in crashes between vehicles whose masses differ by more than about 10%.