Nature has programed us to respond to danger in fractions of a second. With the help of cortisol, also called stress hormone, we immediately decide whether to fight or flee. In times long gone by such a decision was part of everyday life and served the purpose of saving our life at the lowest possible expense. This hormone helps mental mechanisms, such as readiness, and muscular activity involved in either fighting or fleeing, but it interferes with other activities such as memory or reasoning. In an evolutionary perspective reasoning or remembering past episodes is a distraction if you are fighting or running away from danger. Cortisol weakens synapses, the structures that allow neurons to connect, and so affects learning. In the modern world we rarely have to face situations where we must either fight or flee. Many moments of so-called danger, such as sitting an exam, actually require good reasoning and memory rather running away or fighting the examiner. Evolution has not kept up with the speed of these recent situations and so we end up responding with cortisol when its effects actually complicate the course of events. In fact, it is common for students to say to their examiner that they have a total blank or that they cannot remember anything. It is as if just before a competition an athlete started producing a substance that gives muscle cramps. Parents and teachers should know that children are sometimes the victims of an inverse doping due to natural and automatically triggered mechanisms. If the family and school environment is pleasant and calm these mechanisms are less likely to be triggered. Schools must also consider biological, and not just academic, aspects of how children respond. Ten minutes meditation or breathing exercises are very useful in countering cortisol effects.