Single, widowed, and divorced people have as high as twice the mortality rate of married people, which is a trend that's held true in studies across countries and time periods. Unmarried peoples' tendencies to die seems to tell us about the relative strength of social bonds, which is supported by similar trends seen among ants, bees, and even cells, described in a fascinating paper in Cornell's quantitative biology archive.
In 1994, French entymologists found that insects in groups of ten live longer than those in groups of one or two. The survivorship curves for ants and termites looked like this, with the y-axis representing the percentage of survivors and the thin solid lines representing solitary bugs:
The same thing, it turns out, happens with cells, which die unless they get signals from their neighbors "telling" them to stay alive. This chart shows how smaller groups experienced more death.
Black = 1 million cells per mL; Blue = 100,000 per mL; Pink = 50,000 per mL
Getting to humans, similarly, mortality rate ratios for single versus married people (blue) and widowers versios married people (red) look like this:
So the effect tends to be stronger in men than women, with young widowers faring worse than older.