Complex sentence: So smokers may be more likely to buy insurance, or may tend to buy larger amounts, than non-smokers, thereby raising the average mortality of the combined policyholder group above that of the general population.
Split into simpler sentences: So smokers may be more likely to buy insurance, or may tend to buy larger amounts, than non-smokers. The average mortality of the combined policyholder group will be higher than the average mortality of the general population.