单项选择题

Marriage was one of the first non-biological factors identified as improving life expectancy. The explanation given was that married people tend to take fewer risks with their health and enjoy better mental and emotional health. Marriage also provides more social and material support.
The difference between married people and single people, in terms of health, is narrowing. To really understand this, we have to be precise about terms. Researchers typically distinguish between never married, currently married, widowed and divorced. As we look in-depth into this issue of marriage and health, we’ll see that things get pretty fuzzy nowadays.
No one is saying that having a piece of paper that says "married" on it is going to improve your life expectancy. There is something about people who live in marriage that improves life expectancy—or to be more precise, there was something about people who lived in marriage in the 70s that was found to improve life expectancy. Now, people could be listed as "single never married" in census data, but be living with someone and be experiencing all the health benefits of marriage without having the marriage certificate. This complicates research on marriage and health.
Even using the traditional categories of "currently married" or "never married," singles are catching up, but only men. Men who were never married typically had the lowest life expectancy (in 1972). Now, the never married men are closing in on their currently married counterparts. The difference in life expectancy is becoming smaller because that single men now have access to support and health resources that only came because their wife took care of them. In other words, in the 1970s, married men had the advantage because they had their wives to make sure they went to the doctor and took care of themselves. Now, men are taking more responsibility for their own health and take action.
Compared to 1972, people who are widowed now report poorer health than their married counterparts. In the 70s, they reported their health as the same as married people, now their health is about 7% worse. No one really knows why widowhood is more damaging to health now than in the 70s. My guess is that widowed people in the 70s had more of a community and extended family to help them out. Now, the widowed are more likely to be isolated. The central idea of this passage is that ______.