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If you went to an SAT testing site and could run physiological and neurological scans on the teenagers milling outside the door right before the exam, you would observe very different biomarkers from student to student. Those standing with shoulders hunched, or perhaps rubbing their hands, stamping their feet to get warm, might be approaching what Wendy Berry Mendes and colleagues call a “threat state.” According to Mendes, an associate professor of psychology at the University of California, San Francisco, the hallmark of a threat state is vasoconstriction — a tightening of the smooth muscles that line every blood vessel in the body. Blood pressure rises; breathing gets shallow. Oxygenated blood levels drop, and energy supplies are reduced. Meanwhile, a rush of hormones amplifies activity in the brain’s amygdala, making you more aware of risks and fearful of mistakes.

At that same test center, you might see students shoulders back, chest open, putting weight on their toes. They may be in a “challenge state.” Hormones activate the brain’s reward centers and suppress the fear networks, so the person is excited to start in on the test. In this state, decision making becomes automatic. The blood vessels and lungs dilate. In a different study of stress, Jamieson found that the people told to feel positive about being anxious had their blood flow increase by an average of more than half a liter per minute, with more oxygen and energy coursing throughout the body and brain. Some had up to two liters per minute extra.