The stress of a spelling bee or a challenging science project can enhance a student's focus and promote learning. But the stress of a dysfunctional or unstable home life can poison a child's cognitive ability for a lifetime, according to new research. Those studies show that stress forms the link between childhood adversity and poor academic achievement, but that not all adversity--or all stress--is bad for students. Research from Harvard University's Center on the Developing Child in Cambridge, Mass under Dr. Jack P. Shonkoff and from other experts finds that positive stress--the kind that comes from telling a toddler he can't have a cookie or a teenager that she's about to take a pop quiz--causes a brief rise in heart rate and stress hormones. A jolt can focus a student's attention and is generally considered healthy. Similarly, a child can tolerate stress that is severe but may be relatively short-term--from the death of a loved one, for example--as long as he or she has support. "Adults help children develop strategies to help cope with these stressors," Dr. Shonkoff said. "Whether it's reading or managing stress, adults provide the scaffolding for children to build those skills themselves."