by Rosemary L. Hopcroft (@rlhopcro)
There is abundant evidence that marriage is associated with a plethora of positive outcomes for both partners, including better physical and mental health. But skeptics rightfully question whether it is marriage itself that creates these positive outcomes, or whether it is simply that people who are selected as marriage partners are physically and mentally healthier than people who are not selected.
One way to find out if the positive effects of marriage are causal is as follows: imagine you have two identical twins who are alike in all possible ways—genetically (they share 100 percent of their genes) and in terms of their family environment growing up. This means that the twins are likely to be physically and mentally very similar. One twin marries, the other does not, and the married twin is found to be in better physical and mental health than the single twin. This would be powerful evidence that it is marriage in and of itself that causes better physical and mental health, and it is not just the case that physically and mentally healthy individuals are more likely to marry.
