It is in this context that “natural experiments” come into play. A “natural experiment” is a situation that exists in the world, unintentionally, that somehow separates or cleaves correlated exposures. For example, you may have read the recent news story about the IRS mailing letters reminding people of the fee they would incur if they did not sign up for health insurance under the Affordable Care Act (the so-called Individual Mandate). Except somehow the IRS only mailed this letter to a random set of the people it should have. This allowed researchers to test the effect of being sent a letter (versus not being sent one) on later health outcomes. … [Read more...] about What Can Twins Teach Us About How Stress Impacts Health?