Why some people shake legs or bite nails repetitively?

Why do people shake legs, bite nails or generally fidget around when engaged in deep thoughts? Is it a negative feature connected to stress, tension, boredom, etc., as the current science portrays? While explaining the same, I also explain the core factor in the habits of smoking/chewing tobacco, chewing gum, eating popcorn while watching movies, etc.

Degree of Attention

Because of systematic storage and linking of information in the brain1, modifying the degree of attention by evaluating how significant each ongoing interaction is and proportionately allotting and utilizing the allotted amount of resources (as explained in the previous post Degree of Attention) takes very little time and thus, is not a problem in most of your daily interactions.

It is only when you are in the process of thinking continuously, may it be deliberating between multiple decisions or watching a movie, which are of dynamic nature and typically involve one or multiple chains of thoughts, the time it takes for rescaling and using the processing resources (a period I call “rescaling lag”) can eat up into the time the brain requires to attend to successive thoughts or inputs, which may result into missing out on processing important parts of such interactions.

For example, when watching a movie, such rescaling lag can result into missing out on critical parts of the story of a movie if they are presented in quick succession. The same is the reason why many people, when watching the same movie again, find new aspects to its story.

Based on the optimizing aspect of the evolutionary process, the human brain has evolved with a remedy for such a limitation, as explained below.

Degree of Attention

The conscious part of the brain initiates the execution of a physical process, usually a simple and repetitive process like shaking legs, biting nails, pacing around, or generally fidgeting, and transfers it to the non-conscious part of the brain. As it is repetitive, it does not need conscious intervention. Doing so allots a certain amount of processing resources to such repetitive process.

Whenever the conscious part of the brain evaluates the ongoing interaction to be significant and needs to utilize more resources to process it, it switches utilization of resources from the repetitive process mentioned above to such significant interaction.

Doing so drastically reduces the rescaling lag, the reason for which is switching of processing from one task to the other is almost instantaneous, as it saves on time taken in allotting and utilizing processing resources, which are more time consuming as compared to evaluating the ongoing interaction.

Fidgeting Behaviour

This way, the brain pre-allots processing resources to a simple and repetitive task which is governed by the non-conscious part of the brain and switches such pre-allotted resources to process significant part(s) of the ongoing interaction as and when required, thereby saving on rescaling lag.

Saving on rescaling lag is the core factor in the habits of smoking tobacco, chewing gum, eating popcorn while watching movies, etc.

Unfortunately, the current science does not recognize such an efficient feature of the human brain and treats it as something that is connected to stress, tension, boredom, etc. Some scientists also treat it as a disorder, which could be based on the fact that patients with disorders like autism, schizophrenia, etc. who have problems in rescaling their processing resources also engage in repetitive behaviour.


Note: This post sufficiently explains the mechanism of degree of attention and the logic behind fidgeting behaviour like shaking legs, biting nails, tapping feet, etc. Please follow the links given below only when you want to dig deeper into these subjects.

(Source – Mechanisms of the Mind by Parag Jasani)

1 more under title Dichotomized Operating System (DOS) – Introduction – page 19

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