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Contrafreeloading is a behavioural phenomenon in which animals choose to work for food (e.g. by pulling a lever) despite there being an easily accessible and free source of identical food available nearby. Contrafreeloading behaviour is seen in a range of organisms, including humans.[2] Whilst contrafreeloading is a replicable phenomenon, no single explanation of the behaviour has yet been conclusively determined. However, several factors which effect whether an organism will contrafreeload and the rate at which this occurs have been established. One determinant of the level of contrafreeloading is the level of hunger experienced by the organism, with contrafreeloading decreasing as hunger increases. Other factors which impact on the level of contrafreeloading include the level of stimulus change that occurs during the supply of food, prior training, the effort required to receive the food and the level of familiarity with the environment. As working for food in the presence of a free source can be seen as an anti-economical behaviour, a feature common in compulsive actions, contrafreeloading has been used as a model of compulsive-disorders in clinical trials.

History
Contrafreeloading was first coined and examined by G.D. Jensen in 1963. In the study, 200 rats were trained to pull a bar when they desired food. This would release food into a dish. Once trained, an identical source of freely available food was placed nearby. Despite the bar releasing food at an increasingly infrequent rate, all but one of the rats continued to pull the leaver to receive food rather than eating from the nearby free source. Building on this finding, subsequent studies aimed to establish whether this behaviour was species-specific. These studies established that the behaviour was not limited to rodents, but rather a wide variety of different species. Most recently, these studies have been developed and adapted for use in the pharmaceutical industry where it is now used as an analog of compulsive disorders.

Factors Impacting Contrafreeloading
Several factors impact on whether an organism engages in contrafreeloading and the rate at which it occurs.

Prior Training
Prior training was first observed as a significant driver in the rate of contrafreeloading in Glen Jensen's seminal study. The study found that in trials in which both free food and lever-operated food were available, the amount of pellets the rats would eat from the lever-operated dispenser was positively correlated with the amount of times they had been rewarded on the lever-operated dispenser in the training phase (a phase used to teach the rats how to operate the lever). While further studies support this observation,  an alternative explanation for the findings proposes that prior training does not increase rates of contrafreeloading directly. This proposal states that laboratory rats used in contrafreeloading studies are neophobic and will choose food from the container most familiar to them. Thus, prior training does not increase contrafreeloading via training effects, but by increasing familiarity with the food container. This notion has been challenged by numerous studies which controlled for exposure to both food containers yet still observed contrafreeloading.

Deprivation Level
The effect that food deprivation has on rates of contrafreeloading was first examined by Tarte & Snyder. Their experiment compared groups of rats which had been deprived of food for varying amounts of time, ranging from 0 hours to 92 hours. During the final feeding session, it was found that rats which had been deprived of food for a longer period took more earned food than those which had been deprived for shorter periods of time, indicating that increased hunger might be a driver of increased contrafreeloading. Whilst the study indicates that increased hunger increases contrafreeloading, the large amount of food that the food-deprived animals earned, yet failed to eat, indicates that increased hunger might not be the only driver of the behaviour.

Effort Required
A 1970 study by Carder & Berkowitz sought to ascertain how varying the amount of effort required to receive the food impacted on the rate of contrafreeloading. It was found that increasing the amount of lever pulls required to receive the food reduced the rats preference for the earned food. When two lever pulls were required to receive the earned food, the rats preferred earned food over free food 80% of the time. Increasing the amount of lever pulls required to ten pulls decreased the preference of earned food to 30%. This indicates a preferences for earned food as long as the effort required is not too high. Based on this finding, Mitchell et al. (1982) challenged the notion that the free-food provided was actually free. This was based on the notion that previous studies had not previously controlled for the effort required to get to the free-food container -- a free food bowl which was hard to access might not be perceived as requiring more effort than pulling a lever of a nearby dish. It was found that placing the free-food container in an area which required the rats to climb increased the rate of contrafreeloading. This indicates that the effort required to acquire both earned food and free food impact on rates of contrafreeloading.

Stimulus Change
The change of stimulus that occurs when the earned food is released or presented was first examined as a factor impacting on contrafreeloading by Alferink et al. (1973). The study established that minor changes in stimulus, such as a light changing colour when the earned food is released into a container bowl, can impact on the rate of contrafreeloading. In both rats and pigeons, it was found that the rate of contrafreeloading decreased if a light which typically flashed when food was released in the training phase was deactivated during the experimental phase.

Environmental Uncertainty
The effect environmental uncertainty has on contrafreeloading was first examined by Havelka in 1956 using rats. The study aimed to examine whether varying the site of the earned-food dish would promote or impede rates of contrafreeloading. Using a large cage with 8 rooms allowed for the earned-food dish to be hidden in different rooms on different trials, whilst the free-food dish remained in the same room every trial. It was found that when the site of the earned-food dish was varied, the rats would prefer seeking for the earned-food dish, with the preference not diminishing as long as the site of the dish continued to change. However, fixing the location of the earned-food dish did slightly diminish the rate of contrafreeloading, although the effect remained. The preference for uncertainty whilst foraging can also be seen when the food is changed, rather than the location of the dish. In a 1994 study, Inglis & Shepherd (1994) found that rats continued to work for food that they knew to be contaminated and cause sickness, despite their being a non-contaminated dish of food freely available. One proposed explanation for this behaviour is that it allows animals to continuously gather information regarding the quality of the food and monitor whether the source, once known to produce healthy food, returns to its original wholesome state.

Rearing Conditions
One factor which must be addressed when using laboratory-reared animals for research into contrafreeloading is the method in which they were reared. Animals which have been brought up in bland, sensory-deprived environments are more exploratory when presented with a novel situation than sensory-enriched rats, the differences in rearing styles and therefore behaviour might impact on rates of contrafreeloading. A 1981 study by Nau et al. addressed this issue by comparing three different groups of rats in an identical contrafreeloading task, with each of the groups having received varying amounts of sensory enrichment during their upbringing. It was found that rats which received the least sensory enrichment during their rearing period were more stimulation-seeking and also exhibited higher levels of contrafreeloading. This highlights the effect that prior experience can have on contrafreeloading and indicates that rearing conditions are an important variable which must be controlled when carrying out contrafreeloading experiments.

Species Which Exhibit Contrafreeloading

 * Homo Sapiens - Adult
 * Homo Sapiens - Children
 * Pan Troglodytes - Chimpanzee
 * Macaca Mulata - Rhesus Macaque
 * Meriones Unguiculatus - Mongolian Gerbil
 * Rattus Rattus - Wild Black Rat
 * Rattus Norvegicus - Wild Brown Rat
 * Sturnus Vulgaris - Starling
 * Corvus Brachyrhynchos - American Crow
 * Gallus Gallus Domesticus - Domestic Fowl
 * Columba Livia - Laboratory Pigeon / Rock Dove
 * Betta Splendens - Siamese Fighting Fish

Clinical Use
The clinical use of contrafreeloading developed in response to a need to reliably induce compulsive disorders, in order to develop pharmacological treatments. Whilst contrafreeloading is not in itself considered a compulsive disorder, it has been found that the use of dopaminergic agonists increases the severity of contrafreeloading to rates which share key features with the major symptoms of compulsive disorders. Specifically, the application of dopaminergic agonists results in the organism continuing to contrafreeload regardless of the behavioural cost or physical exertion required. This finding allows researchers to induce a state of contrafreeloading severe enough to act as a model of compulsive disorders. Consequently, pharmaceuticals which reduce rates of this induced contrafreeloading/compulsive disorder might be of interest when attempting to treat non-induced compulsive disorders. Research using this technique has found that Clomipramine, a serotonin re-uptake inhibitor, can decrease rates of contrafreeloading.