What are some reasons females outlive men in the United States and most countries of the world?

While life expectancy continues to increase, differences exist between men and women. As a general rule, women outlive men, yet there are a few countries where men survive longer than women.

1. What are some reasons females outlive men in the United States and most countries of the world?
2. What are some reasons that men outlive women in countries other than the United States?
3. What are some of the factors that contribute to these gender differences?