The Domino Theory

Heinrich's Domino Theory states that accidents result from a chain of sequential events, metaphorically like a line of dominoes falling over. When one of the dominoes falls, it triggers the next one, and the next... - but removing a key factor (such as an unsafe condition or an unsafe act) prevents the start of the chain reaction.

What are Unsafe Conditions and Acts?
According to Heinrich, all incidents directly relate to unsafe conditions and acts, which he defines as “unsafe performance of persons, such as standing under suspended loads ... horseplay, and removal of safeguards”; and “mechanical or physical hazards such as unguarded gears ... and insufficient light.” These have been described in details in human behaviour and errors in Module 7.

The Dominoes
Heinrich posits five metaphorical dominoes labelled with accident causes. They are Social Environment and Ancestry, Fault of Person, Unsafe Act or Mechanical or Physical Hazard (unsafe condition), Accident, and Injury. Heinrich defines each of these "dominoes" explicitly, and gives advice on minimizing or eliminating their presence in the sequence.

Social Environment and Ancestry:
This first domino in the sequence deals with worker personality. Heinrich explains that undesirable personality traits, such as stubbornness, greed, and recklessness can be "passed along through inheritance" or develop from a person's social environment, and that both inheritance and environment (what we usually refer to now as "nature" and "nurture") contribute to Faults of Person.


Fault of Person:
The second domino also deals with worker personality traits. Heinrich explains that inborn or obtained character flaws such as bad temper, inconsiderateness, ignorance, and recklessness contribute at one remove to accident causation. According to Heinrich, natural or environmental flaws in the worker's family or life cause these secondary personal defects, which are themselves contributors to Unsafe Acts, or and the existence of Unsafe Conditions.

Unsafe Act and/or Unsafe Condition:
The third domino deals with Heinrich's direct cause of incidents. As mentioned above, Heinrich defines these factors as things like "starting machinery without warning ... and absence of rail guards. " Heinrich felt that unsafe acts and unsafe conditions were the central factor in preventing incidents, and the easiest causation factor to remedy, a process which he likened to lifting one of the dominoes out of the line. These combining factors (1, 2, and 3) cause accidents.


Heinrich defines four reasons why people commit unsafe acts "improper attitude, lack of knowledge or skill, physical unsuitability, [and] improper mechanical or physical environment." He later goes on to subdivide these categories into "direct" and "underlying" causes. For example, he says, a worker who commits an unsafe act may do so because he or she is not convinced that the appropriate preventative measure is necessary, and because of inadequate supervision. The former he classifies as a direct cause, the latter as an underlying cause. This combination of multiple causes, he says, create a systematic chain of events leading to an accident.

Heinrich says, "The occurrence of a preventable injury is the natural culmination of a series of events or circumstances which invariably occur in a fixed and logical order." He defines accidents as, "events such as falls of persons, striking of persons by flying objects are typical accidents that cause injury."


Injury results from accidents, and some types of injuries Heinrich specifies in his "Explanation of Factors" are cuts and broken bones.

To be fair to Heinrich, he does insist that "the responsibility lies first of all with the employer." Heinrich specifies that a truly safety-conscious manager will make sure his "foremen" and "workers" do as they told, and "exercise his prerogative and obtain compliance ... follow through and see the unsafe conditions are eliminated." Heinrich's remedy for such non-compliance is strict supervision, remedial training, and discipline.

Heinrich's Domino Model of Accident Causation


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