Herbert W. Heinrich was a pioneering occupational safety researcher, whose 1931 publication Industrial Accident Prevention: A Scientific Approach1 was based on the analysis of large amounts of accident data collected by his employer, a large insurance company. This work, which continued for more than thirty years, identified causal factors of industrial accidents including “unsafe acts of people” and “unsafe mechanical or physical conditions”.
Heinrich is most famous for originating the concept of the “safety pyramid”. He also developed the “five domino model” of accident causation, a sequential accident model which represents an accident sequence as a causal chain of events, represented as dominos that topple in a chain reaction. The fall of the first domino leads to the fall of the second, followed by the third, etc.2, as illustrated below.
Heinrich saw the occurrence of a “preventable injury” as the culmination of a series of events that form a sequence, similar to a row of dominos placed so that the toppling of a first domino knocks down the next, which makes the third fall down, and so on until the entire row is toppled. If this series is interrupted by the elimination of even one of the several factors that comprise it, the injury will not occur, as shown in the figure below:
In the first version of this model, published in 1931, the five factors identified were:
domino 1: ancestry and the worker’s social environment, which impact the worker’s skills, beliefs and “traits of character”3, and thus the way in which they perform tasks
domino 2: the worker’s carelessness or personal faults, which lead them to pay insufficient attention to the task4
domino 3: an unsafe act or a mechanical/physical hazard, such as a worker error (standing under suspended loads, starting machinery without warning…) or a technical equipment failure or insufficiently protected machinery
domino 4: the accident
domino 5: injuries or loss, the consequences of the accident
Over time, the idea of attributing workplace behaviour to ancestry and to ingrained personal faults was found to be inappropriate, and more recent versions of the model replace the labelling of the first two dominos by aspects related to planning, work organization and leadership, or more generally management’s control of organizational factors of safety.
This theory of accident causation was later further developed by Frank Bird, who improved the description of managerial “dominos”, and who generalized the last “accident” domino to cover any loss (lost production, damage to equipment or other assets, and not only injuries).
This linear accident model is simple and easy to understand. Compared with the very simplistic analyses that were common at the time (“accident caused by worker error”), it helped managers to think about and identify underlying causal factors that could contribute to accidents. Its promise of allowing the interruption of the accident sequence by acting on underlying causal factors (“pulling out a domino”) helps to convince people to adopt the corrective actions suggested by the accident investigation.
However, the model can contribute to a focus on the search for culprits in the accident sequence, rather than on a detailed understanding of all the factors that may have contributed to the accident.
The domino model is widely seen today as being much too simplistic to be a useful tool to help understand the causal factors of accidents:
it leads to an excessively simple view of the contribution of human performance to accidents
it adopts a purely linear and mechanical model of causality, which is inappropriate in complex systems where accidents are generally caused by many interacting, partially competing and unpredictable factors
Heinrich, H. W. 1931. Industrial accident prevention: a scientific approach. New York: McGraw-Hill.↩
A similar analogy was in use in geopolitical theory during the Cold War, speculating that if one country “fell” to communism, then surrounding countries might quickly follow. This domino theory was used by the US administration to justify the need for American intervention in various parts of the world.↩
Heinrich wrote that “Recklessness, stubbornness, avariciousness, and other undesirable traits of character may be passed along through inheritance”. This kind of belief is not very different from that held by supporters of eugenics.↩
During the period 1920–1960, a number of industrial psychologists put forward a theory that certain workers are more “accident-prone” than others (they are more likely than others to sustain accidents, even though exposed to equal risk). Some people working in high-risk industries still hold this belief. However, research since the 1960s shows that this theory has little validity. Some population categories do tend to have more accidents than others (for instance, young male drivers tend to have more car accidents than older drivers), but these factors concern a category of people (age, level of experience, level of education, for example) rather than a specific individual. Organizational and workplace factors have a greater impact on the occurrence of accidents than factors related to the individual. Finally, any theory concerning accidents that leads to the allocation of blame on individuals has many negative side-effects for safety, such as encouraging defensive reactions by individuals that strongly reduce the reporting and sharing of safety information. For these reasons, “accident-proneness” theory is not a useful concept for safety management.↩