A very readable overview of recent research from PLoS Biology: "When resources are short, growing organisms face an existential choice: should you ignore the shortage and hope for better times soon, or scale back and live within your limited means? And if you do scale back, will there be any payoff later in life? For animals, these choices are played out hormonally, with environmental fluctuations leading to internal rearrangements in endocrine signal and response throughout the growing body. In mammals, two principal hormones - growth hormone (GH) and insulin-like growth factor 1 (IGF-1) - promote growth. Remarkably, inhibiting one or both of these two not only retards growth, but also extends lifespan, not just in lab animals, but possibly also in people: mutations that reduce the function of the IGF-1 receptor have recently been discovered in centenarians (who are also short). Growth occurs throughout the body, and receptors for IGF-1 are found in every organ on virtually every cell. But [researchers have now shown] that it is the IGF-1 receptors in the brain that set the pattern for growth and lifespan."