Research the article: Keinan G (2002) The effects of stress and desire for control on superstitious behaviour. Personality and Social psychology Bulletin 28, 102-108
For many people, magical or superstitious thinking can bring comfort in times of stress. This study was done to see how many people would "knock on wood" for luck when asked about their health or good fortune. The subjects would sit at a wooden table and answer several questions about various issues by the interviewer. Whenever a question regarding their health or good fortune was asked, it was determined that more people actually did "knock on wood" for those specific questions.