Were Herbert William Heinrich’s theories valid? Do they still matter?
Old worker photo

Examining the foundation

Were Heinrich’s theories valid, and do they still matter?

October 1, 2011


  • Heinrich's most famous theories include unsafe acts of persons are responsible for most accidents and the 300-29-1 ratio of workplace accidents.
  • Critics claim that adhering to the Heinrich model can lead to an over-emphasis on worker behavior and not enough attention on systems.
  • Heinrich is credited with bringing attention to workplace safety and focusing on the human element of safety.

Editor’s Note: This article retains the use of the term “accident” instead of “incident,” as well as the phrase “unsafe acts of persons,” to be consistent with the terminology Heinrich used in his book.

Safety professionals generally agree that Herbert William Heinrich had a significant impact on the practice of safety, but whether his influence was positive or negative remains a subject of debate.

“He was a pioneer in bringing attention to workplace safety; however, to describe what he did as ‘research’ is questionable,” said Judith Erickson, president of Plano, TX-based safety consulting firm Erickson Associates.

Heinrich is best known for his 1931 book, “Industrial Accident Prevention: A Scientific Approach,” in which he said 88 percent of accidents are caused by “unsafe acts of persons” and put forth what often is referred to as Heinrich’s accident triangle or pyramid: In a group of 330 accidents, 300 will result in no injuries, 29 will result in minor injuries and one will result in a major injury.

Neither premise sits well with Michael Taubitz, founder and principal consultant of Lean Journey LLC in Fenton, MI. He called Heinrich’s views a “constraint” on safety professionals who are trying to attack high-severity, low-probability incidents.

“The Heinrich myth prevails and we do little as a profession to dispel myths,” Taubitz said.

Bob LoMastro, a longtime safety trainer specializing in electrical safety, offered a different view. “I think everybody’s taking Heinrich too literally and they’re forgetting his big picture, and the big picture is the vast majority of accidents are forecasted before a big accident occurs – we’ve had opportunities to correct the hazards, but did not,” LoMastro said.

Wrapped up in this issue are questions about Heinrich’s data and conclusions, and if theories espoused 80 years ago even apply today. 

Basic principles

Heinrich was an assistant superintendent in the engineering and inspection division of Travelers Insurance Co. in Hartford, CT. For his book, he reviewed 75,000 injury and illness cases – 12,000 from insurance records and 63,000 from plant managers – as well as actuarial and engineering reports.

The book introduces 10 “axioms of industrial safety,” the first of which states: “The occurrence of an injury invariably results from a completed sequence of factors – one factor being the accident itself.” That sums up Heinrich’s domino theory. He believed the following five factors must be present for an accident to occur:

1. Ancestry and social environment
2. Fault of person
3. Unsafe act or mechanical or physical hazard
4. The accident
5. The injury
“Unsafe act or mechanical or physical hazard” lines up with Heinrich’s third, and arguably most controversial, axiom: “The unsafe acts of persons are responsible for the majority of accidents.” According to Heinrich, 88 percent of accidents are caused by unsafe acts of persons and 10 percent by unsafe machines (with 2 percent being unavoidable).

Decades later, Heinrich’s theories – including the domino theory and accident triangle – continue to draw followers and critics. In his 2002 book, “Heinrich Revisited: Truisms or Myths,” published by the National Safety Council, Fred A. Manuele questioned the validity of the 300-29-1 ratio, noting that Heinrich revised it without explanation in subsequent editions of his book.

Another issue Manuele raised was that the original files Heinrich used do not exist, preventing others from reviewing his work.

James Howe, head of Safety Solutions in Medford, OR, and former assistant director of health and safety for the United Auto Workers, also takes issue with the ratio.

“The pyramid theory has really done a disservice to the safety profession,” he said, “because it has misled people running safety programs into thinking that if they work on minor incidents, major incidents will go away. And many, many companies are aware that that is not the case.” In fact, he said, certain companies with award-winning low injury rates have suffered some of the worst catastrophic incidents during the past 10 years.

In refuting Heinrich’s work, Manuele singled out the 88-10-2 ratio of accident causation as having the most influence and causing the most harm to the safety profession.

“Why harm? Because when basing safety efforts on the premise that man failure causes the most accidents, the preventive efforts are directed at the worker rather than on the operating system in which the work is done,” Manuele wrote.

In addition, Howe said attributing 88 percent of accidents to unsafe acts fails to take into account that accidents usually have multiple causes and contributing factors.

‘Accurately reporting bad data’

Heinrich’s work forms the basis for behavior-based safety, an approach that focuses on identifying and changing unsafe worker behaviors. Critics claim BBS emphasizes the worker without taking the system into account, a common argument aimed at Heinrich’s theories.

Statistician William Edwards Deming, whose work on quality control is credited with helping to turn around the Japanese economy after World War II, believed the vast majority of problems were the fault of the system (or management), not the worker.

Erickson holds that view as well. She cited a 2010 study from the National Opinion Research Center at the University of Chicago that found 85 percent of workers rated workplace safety as the most important labor standard. The findings fit with Erickson’s observations as a consultant and safety manager, yet she was struck by the dichotomy between that and what Heinrich said.

As a proponent of evidence-based interventions, Erickson’s main critique is that Heinrich’s work was not scientific. He used accident reports completed by supervisors, based his observations on retrospective data and failed to investigate possible underlying causes, she said.

“All he provided was a conclusion, and he primarily relied on the subjective observations of others.” Some of whom “had a stake in blaming the employee rather than assuming any responsibility,” Erickson added.

Howe echoed that concern. “I do think what people don’t realize is that it was flawed data to begin with,” he said. “I don’t think he miscounted reports or was misreporting the data. I think he was probably accurately reporting bad data. I think that’s the problem. I think it has not served us well. It has hurt the profession an awful lot.”

‘No longer applies’

Taubitz began questioning Heinrich’s model when he worked as global safety director for General Motors in the late 1980s. He and his colleagues noticed that the exposures causing fatalities had nothing to do with sprains, strains or other reportable injuries, “and we intuitively understood that the Heinrich model didn’t fit,” he said.

But when they took their case to senior management, Heinrich’s triangle “was a huge barrier to try to open eyes that something different is happening,” Taubitz continued. “The belief that if we drove minor injury rates almost to zero that somehow magically we would eliminate fatalities was a pervasive thought in the industry.”

That belief is “just wrong” and hinders efforts to advance safety and risk reduction, according to Bruce Main, president of Design Safety Engineering Inc. in Ann Arbor, MI. 

He said Heinrich’s work encourages people to look strictly at procedures and training instead of rethinking system design. He pointed out that the design may encourage unsafe acts, such as if a worker has to defeat an interlock system to go in and clear a machine.

“I think, fundamentally, Heinrich’s ideas were useful at the time, but I think it’s time for us to completely debunk Heinrich’s theories and ideas and start fresh,” Main said. “I’ve seen Heinrich’s ideas abused many times, and I’d like this whole idea to be given its fair due and then dismissed because it no longer applies.”

As an example of abuse, he recalled an insurance broker telling management, as part of a sales pitch, that 85 to 95 percent of accidents are caused by unsafe acts of persons. Main’s concern is that such claims let upper management off the hook – they can attribute accidents to unsafe acts and only buy insurance coverage. 

Likewise, Canada-based quality management consultant Wayne Pardy said some consultants practice what he termed “parrot-based safety” – they repeat unproven numbers based on Heinrich’s work to promote their solutions.

“When an idea has come to the point where nobody questions it, you lose that balance. I think we’ve lost a little bit of that balance,” Pardy said.

Redeeming qualities

A common charge against Heinrich is that he blamed the worker for workplace accidents. But LoMastro said critics are interpreting Heinrich’s theory the wrong way.

“The fact that most accidents result in unsafe behavior doesn’t mean the worker did it on purpose,” he said, adding that a worker may have been given poor training or improper tools.

Indeed, while Heinrich said unsafe acts of persons caused most accidents, his eighth axiom states: “Management has the best opportunity and ability to prevent accident occurrence, and therefore should assume the responsibility.” 

LoMastro also dismissed criticisms about the specific numbers in Heinrich’s book. “Heinrich’s theory was only a theory. It was never meant to be statistical data, so to speak,” he said.

Erickson noted that Heinrich’s work reflects the prevailing attitude of his times. Before him, Frederick Taylor had developed the concept of scientific management to increase industrial efficiency and productivity, and Henry Ford had started using the assembly line to manufacture cars. As Erickson put it, workers were viewed as a cog in the wheel.

Although not a fan of Heinrich’s beliefs, Pardy gives him credit for focusing on the human side of safety. Nevertheless, he said that the workplace has changed dramatically since 1930. Employers face an aging workforce and young workers with different values and expectations, making Heinrich’s work a starting point rather than the end.

“I think what Heinrich’s book did in the ’30s, it gave us the foundation,” Pardy said.

Value of research

If Heinrich was wrong, then what is the correct model for incident prevention?

Taubitz said the profession does not have a perfect model to help forecast severe accidents and fatalities, but he recommended a task-based approach. Main, also a proponent of task-based risk assessments, suggested replacing Heinrich with “prevention through design.”

Beyond Heinrich, Erickson called research “pivotal” in ensuring that employers do not waste time, money and effort on unproven interventions.

Pardy agreed. “I think what we need is more objective evaluation, more independent research into workplace safety and health management,” he said.

While not downplaying the importance of research, Main cautioned against arguing about the probability of a hazardous event occurring. “When you do risk assessments quite a bit, you figure out that there is often much less controversy over how we’re going to reduce the risk than there is over the ratings,” he said. “If you can get over that hump and focus on risk reduction, you can avoid the discussions and all the hand-wringing.”

In Howe’s opinion, time-crunched safety professionals often fail to take full advantage of available research. “Safety research can be very critical, and it’s undervalued in our profession,” he said. “There are a lot of important questions that have not been researched. There’s a huge opportunity in safety research. We should all be paying more attention to it.”