Shortly after Harvard psychologist Daniel Gilbert's book, Stumbling on Happiness, was released, he was on TV, where he informed Stephen Colbert that "marriage is one of the best investments you can make in happiness." That advice implicitly rests on a causal claim: marriage causes happiness.

Much recent research documents a positive correlation between marriage and happiness. But is the relationship causal?

(a) Provide an argument for why the correlation between marriage and happiness might be the result of reverse causation (happiness causing marriage, rather than the other way around).

(b) Identify two confounders that you think might make a causal interpretation of the correlation between marriage and happiness problematic. For each, explain why you believe the confounder might affect both treatment (being married) and outcome (happiness).

(c) Sign the bias for each of the confounders you identified. Having done so, explain whether each tends to make the observed correlation between marriage and happiness an over- or under-estimate of the true causal effect.