Chapter 4 discusses Heinrich’s ideas and behavior-based safety. Heinrich reestablished the idea that many accidents and injuries are preventable. He used a metaphor of a row of dominoes to explain how distant causes lead to injuries. Ancestry and the social environment give rise to character flaws in a worker, such as bad temper, ignorance, or carelessness. Character flaws give rise to unsafe conditions, mechanical hazards, and unsafe acts. These factors in turn lead to accidents, which lead to injuries and fatalities. Like a row of falling dominoes, Heinrich suggested that the sequence could be interrupted by removing the right factor in the sequence. Heinrich’s opinion on the best point for intervention shifted throughout his career. Early on, he placed a strong emphasis on improving the physical conditions and physical safeguards of work. Later he placed increasing emphasis on eliminating unsafe acts by workers. He advocated creating an environment where even small undesirable acts are not tolerated. It was mostly through these later, human-error-focused ideas that Heinrich influenced the theory and practice of safety. Behavior-based safety is one of the most visible expressions of it, with us to this day.Three key ideas of Heinrich’s have influenced safety practices (and even some theories) for decades: (1) Injuries are the result of linear, single causation; (2) there is a fixed ratio between accidents (or simply “occurrences”), minor injuries and major injuries; and (3) worker unsafe acts are responsible for 88% of industrial accidents. All three have been proven false.