The definition of Yerkes-Dodson Law refers to “the inverted U curve which is an ‘iconic symbol’ or a model for arousal and stress towards human performance [Wickens & Holland (2000 9)].
The definition by the Psychology Dictionary  refers to the Yerkes-Dodson Law as “The proposition that optimal task performance occurs at an intermediate level of arousal, with relatively poorer performance at both lower and higher arousal levels, leading to an inverted U relation between arousal and performance, with the proposition that the peak occurs at a lower level of arousal for easy than for difficult tasks.
The Yerkes Dodson Law was named after US psychologists Robert Mearns Yerkes (1876–1956) and John Dillingham Dodson (1879–1955) who published an article in the Journal of Comparative Neurology and Psychology in 1908 on the ‘Relation of Strength of Stimulus to Rapidity of Habit-formation’, though they did not enunciate the law explicitly or attempt to test it directly. The inverted U curve idealised from the original Yerkes and Dodson data actually relates to arousal, and not to stress [Yerkes & Dodson (190810)]. Over the decades, arousal has been regarded to many as a synonym of stress but in fact ‘arousal is far from being an adequate and accurate indicator of stress’ [Stokes & Kites (1994 7)]. The relationship between arousal and stress proposed by Michael Apter of University College, Cardiff states that stress does not vary directly as a function of arousal level, rather it results when the actual level of arousal does not match the preferred level [Stokes & Kites (1994 7)].
The Yerkes-Dodson Law has the ‘performance level’ on the vertical axis and the ‘arousal level’ on the horizontal axis. Stress level is regarded as acting to the increase of arousal level [Green, Muir, James, Gradwell & Green (1996 3)]. The pattern of performance suggested that at low arousal level, there will be low performance level. Performance level is increased when arousal level increased [Wickens & Holland (2000 9)].
As the level of arousal is increased, it will reach an optimum level in which performance will decrease even though arousal level is high. This is known as the ‘Knee’ in the curve, a second characteristic in the Yerkes-Dodson Law [Wickens & Holland (2000 9)]. However, optimum level of arousal and performance varies for different task. Complex task will have a lower level of arousal and performance compared to simple task. This is because complex task involves greater demands and greater memory loads, hence it is more vulnerable to breakdowns in the process [Thom (1997 8)].
Graph showing the relationship between arousal and performance
Three Levels of Yerkes Dodson Law
To further discuss the theoretical aspects of the Yerkes Dodson Law, the inverted ‘U’ curve can be divided into three different sectors
LOW Arousal Level
LOW arousal level or under-aroused is mostly associated with deep sleep, fatigue, sleep deprivation, a lack of motivation and low body temperature. At low arousal levels, pilots usually do not expect to perform any difficult task, or having low motivation to perform the task, thus human attentional mechanisms will not be particularly active. Hence, pilots will scan the environment slowly without focusing on any particular source of information [Green, Muir, James, Gradwell & Green (1996 3)].
Supporting Evidence in Aviation
On 3rd September 1989 2045hour, VARIG airline flight RG 254 made a forced landing into a jungle near Sao Jose do Xingu, Brazil due to fuel exhaustion [ICAO4]. The flight crew entered into the flight computer 270degrees instead of 027degrees. The plane was 600NM off course. Fuel exhaustion occurred which leads to the forced landing in the jungle. The navigation mistake went unnoticed for 2hours because the flight crews were reported listening to the World Cup Qualification Match between Brazil vs. Chile.
This example demonstrates what serious consequences will happens if pilots are at low arousal level resulting low stress during flight. They are in a state of boredom during the cruise phase of the flight, thus they switch their transmission towards the commentary of the world cup soccer match. With low stress among the pilots, they would feel that things are under-control, thus it resulted in the pilots not following procedures to check their bearings. In the end, this low arousal level of pilots creates low stress thus causing navigation error which resulted in an air accident.
OPTINIUM arousal level
Optimum arousal level will create optimum performance. With optimum arousal level, it will create optimum amount of stress for a pilot to achieve its best performance in a particular simple or complex task.
Supporting Evidence in Aviation
During March 1992 at Reading, Pennsylvania USA, an aircraft had a bird-strike and landed safety at the nearest Reading airport despite a damage right wing from the impact of the bird [NASA 5]. The aircraft’s airspeed had drop from 125knts to 105knts with vibrations all over the aircraft. Furthermore, communication with the ATC is unclear and distorted.
This example describes how arousal level and stress will affects one’s performance to either succeed or fail in the critical task. The pilot has a high arousal level from: the impact of the bird, the risk of crashing, vibrations, lost of airspeed, unclear communication with ATC. All these arousal factors results in a high level of stress. Instead of being over-aroused, the pilot managed to handle the stress and find the way towards Reading Airport to land the aircraft. If the pilot was over-aroused, deteriorating in performance will occur, but in this incident, the pilot handled very well in the situation and he even joked with the ATC saying that he had a goose hanging from his starboard wing.
HIGH arousal level (over-aroused)
High arousal level or over-aroused are often associated with fear, panic, under-confidence, lower concentration, over-reacting, physically tense-up, exhausted and difficult in making decisions [Thom (1997 8)]. As arousal level increased higher above its optimum level, high performance will start deteriorating and thus pilots make errors which can cause fatal accidents. Deteriorating performance occurs because as arousal increase, human tents to restrict or focus attention onto a particular event or item which we perceived as the primary demand [Green, Muir, James, Gradwell & Green (1996 3)].
Supporting Evidence in Aviation
An example of pilots associated with high arousal level or over-aroused will be the air accident of Galaxy Airlines, flight 203 at Reno, Nevada, 21st January 1985. The crew of this particular Lockheed Electra flew into the ground while apparently focusing all their attention upon a vibration which they erroneously believed signalled mortal danger and imminent catastrophe [NTSB 6].
The crew were over-aroused by the vibration that they tend to narrow down their attention missing out critical information like the altitude readings in this case. The effects of high stress created by the high arousal level, was over their optimum level, thus the captain fail to control the aircraft, while the co-pilot fails to monitor flight path and airspeed resulting in this fatal accident.
A study carried out by Broadhurst (1959 ) to confirm the Yerkes-Dodson Law on rats showed an acceptable significance. 120 male albine rats were allowed to swim underwater for 0, 2, 4, or 8 seconds. It was found that the optimum learning for the easiest discrimination task occured with the motivation induced by 4 seconds air depriviation.
Way forward (to do list)
Refer to Stress, Stress Managment.
Knowledge Management Space