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The Domino Theory

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The Domino Theory

The following famous quotation presages Heinrich’s Domino Theory, and explains it using a different metaphor (horseshoe nails) than Heinrich (dominoes falling over in a line).

"For the want of a nail, the shoe was lost; for the want of a shoe the horse was lost; and for the want of a horse the rider was lost, being overtaken and slain by the enemy, all for the want of care about a horseshoe nail."
--Benjamin Franklin, Poor Richard's Almanack

Herbert William (“Bill”) Heinrich developed the Domino Theory while working at Travellers Insurance Company in 1929, and expanded on it many times over many years. Heinrich spent quite a few pages elaborating on the Domino Theory in the beginning of his famous book, Industrial Accident Prevention, first published in 1931.

A Short Terminology Note for Heinrich

You will notice that during this section, when we are speaking directly about Heinrich and what he said, we use the term “accident.” Heinrich did not use the term “incident,” so when speaking of Heinrich in specific, it’s more accurate. This terminology also changes within the Domino Theory’s later incarnations, which we will also discuss. Still, when we are speaking in general, we will use the term “incident” as in the rest of this course.

What Is 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.”

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.

1) Social Environment and Ancestry: This first domino in the sequence deals with worker personality. Heinrich explains that undesireable personality traits, such as stubbornness, greed, and recklessness can be “passed along through inheritance” (12) 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.
2) Fault of Person: The second domino also deals with worker personality traits. Heinrich explains that inborn or obtained character flaws (from 1) 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 the existence of Unsafe Conditions.
3) 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 abscence of rail guards.” (12) 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.” (36) 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.
4) Accident: Of accidents, 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.” (12-13) He defines accidents as, “events such as falls of persons, striking of persons by flying objects are typical accidents that cause injury.” (12)
5) Injury: Injury results from accidents, and some types of injuries Heinrich specifies in his “Explanation of Factors” (12) are cuts and broken bones.
To be fair to Heinrich, he does insist that “the responsibility lies first of all with the employer.” (38) (italics in original) Heinrich specifies that a truly safety-conscious manager (of 1950) will make sure his “foremen” and “workers” do as their 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.

The Life and Times of The Domino Theory

By 1950, and the fourth edition of Industrial Accident Prevention, Heinrich was still promoting the same Domino Theory as in 1929. By 1976, however, two scholars named Bird and Loftus were working with Heinrich to update the Domino Theory.

By 1976, the Domino Theory had been updated and changed slightly to reflect new developments in safety theory, and a changing social and political climate. Bird and Loftus (along with input from Heinrich) put forth this new revised version of the Domino Theory in their well-known book Loss Control Management.
By 1994, the Domino Theory was still in use, although it had changed dramatically from Heinrich’s initial postulation. In his textbook Basic Guide to Accident Investigation and Loss Control (1994), Jeffrey W. Vincoli gives a detailed description of this updated Domino Theory, and even seems to use its premises as the basis for the entire text.
By 1994, and Vincoli’s book, the dominoes had been re-labelled and updated (with a new emphasis on management, and incident as property loss), but the basic structure and premises of the theory were still in place. The revised model relabels the dominoes as Management: Loss of Control, Origins/Basic Causes, Symptoms/Immediate Causes, Contact: Incident, and Loss: People – Property.
1) Management: Loss of Control: Vincoli says that “lack of control by management” begins the process that eventually results in incidents. He stresses that if management do their job, which he defines as “planning, organizing, leading, and controlling,” (16) they can prevent incidents from happening at all. If management does not do their job, however, it creates certain basic causes from which incidents arise.
2) Origins: Basic Causes: Vincoli classifies these basic causes as belonging to two different groups: Personnel Factors, and Job Factors. Personnel Factors, he explains, reveal why some people “engage in substandard practices (the third domino)” (or what Heinrich called “unsafe acts”). Job Factors reveal why “substandard conditions” (what Heinrich called “unsafe conditions”) exist.

Personnel Factors

Lack of understanding or ability

Improper motivation (bad attitude)

Illness, mental, or personal (non-work-related) problems

Note that these factors are strikingly similar to, though expressed differently from Heinrich’s “causes for unsafe acts of persons” (IAP, 36).

Job Factors

Inadequate work

Bad design or maintenance

Low-quality equipment

Normal or abnormal wear and tear
3) Immediate Causes: Symptoms: Vincoli brings in unsafe acts and conditions in Domino 3, just like Heinrich. Unlike Heinrich (directly), though, he says that unsafe acts and conditions are symptoms of root causes that dominoes 1 and 2 represent. This approach is slightly different from Heinrich, who seemed to feel the first two dominoes combine to produce the third. Vincoli then goes on to say that in an organizational environment where management allows these factors to continue unchecked, incidents will occur.
4) Contact: Incident: Vincoli defines incidents as any event which has the possibility of creating a loss, and a loss event as an “accident.” (20) He also says that information-gathering (he provides a list of incident types) helps create losses.
5) Loss: People – Property: Using the updated Domino Theory, Vincoli says that losses can’t be predicted, either in how and when they will occur, nor in when. Since his textbook is primarily on controlling loss, he provides several remedies for directly dealing with incidents, mostly derived from Heinrich, and rewritten.

The Persistence of the Domino Theory

Such widely dispersed dates (1929 – 1994) tell of the Domino Theory’s amazing longevity. What makes the Domino Theory so compelling as a causation model? Likely, its popularity arises out of several reasons:

1) It’s simple. Many other causation models, such as Multilinear Event Sequencing, are extremely complex and require the analyst to practically have advanced engineering degrees. (A short note on MES: As the Hitchhiker’s Guide to the Galaxy says: Don’t panic! All information presented on MES in this course will be suitable for those without, because the author’s in the same position.) The Domino Theory is easy to use, assumes a logical, point-to-point view of causality and the world at large, and doesn’t require the investigator to find many, many complicated interrelating causes. It’s also easy for non-safety people, such as management and workers, to understand. Nearly everyone has made lines with dominoes and tipped them over, and nearly everyone likes problems that are easy to understand and fix.
2) It’s an “easy fix.” According to the Domino Theory, if you want to prevent incidents, just remove one of the (metaphorical) dominoes. Such simple solutions usually mean that implementing recommendations generated by this model is no problem, and everybody goes home at the end of the day feeling as though they’ve accomplished something.
3) It places blame. Although one of the things this module strives to teach is that blame is counterproductive, people generally have a “scapegoating” tendency (“find the person responsible and punish them”). Determining who is responsible, and who will take the blame (and the punishment) can feel very good to people who have been hurt (through injury, grief, lost profits, etc.) and who may have a conscious or unconscious desire for revenge.
4) It makes those who use it look good. If an investigator can announce that he or she has “solved” the incident, and knows which one thing or few things to fix in order to prevent the incident from recurring, he or she can improve his or her professional image in a hurry.

This is an edited excerpt from Incident Causation Models, Incident Investigation Course Module 2, for the HSEP Certificate Programme at the University of New Brunswick, written by Sara Stewart with input from Peter Crisp and Don Sayers. (c) Don Sayers and Associates, used with permission.

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