Radical behaviourism is an offshoot of behaviourism which was first described by B.F Skinner. Radical behaviourism is different from earlier ideas of behaviourism because, rather than ignoring internal states like emotions and thoughts, it viewed them as just another type of behaviour, meaning that they could be measured and determined in the same way as observable actions.

One radical behaviourist was Clark Hull, who believed all human behaviour could be explained in terms of physics and maths – he was quoted as saying “a psychologist should not only understand mathematics, but think in mathematics”. Hull’s Mathematico-Deductive Theory was based on creating precise, standardised definitions for factors which affected the probability that a specific Stimulus would lead to a specific Response.

Naming that probability “E”, for Excitation Potential, Hull tried to transform identified factors, such as Inhibition, Fatigue and Motivation, into mathematical functions that predicted people’s behaviour.

This produced mathematical formulae that he believed explained behaviour, such as E = (sHr x D x K x V x J). That formula says: the probability of the Response happening is equal to the number of times the person has been trained to associate the Stimulus with the Response (sHr), multiplied by the person’s motivation to perform the Response (D), multiplied by the size of the Response (K), multiplied by how intense the Stimulus was (V), multiplied by the delay between the Stimulus and Response (J).

Each variable was given a precise operational definition, to aid Hull’s research and replication. Hull hoped to make psychology as scientific and precise in its predictions as physics or chemistry.

However, Hull defined all of these variables so precisely that they often couldn’t be generalised to other situations or even other experiments – this made his work limited meaning it was not widespread in psychology. Hull’s later work was on studying hypnosis, trying to prove the extreme claims of hypnotists. While he did manage to show that things such as hypnotic memory loss existed, he also showed that hypnosis isn’t a special state but a form of suggestion, and some people are more suggestible than others.

The main method of radical behaviourism is Operant Conditioning, which has historically relied on many animal experiments. B.F.Skinner was famous for using a “Skinner box” (though he hated the term). A Skinner box was simply a box containing the animal, a thing that could be changed such as a lever, and a way of delivering a reward, such as a food tube. This invention may sound either fairly pointless or pretty cruel, but was actually neither; it was used in many different types of experiment, and was also operated by Skinner on  a strict “no punishment” policy.

While his animal inventions were well-regarded, he had less luck with inventions for children. His “air-crib”, a temperature and humidity-controlled sleeping area for babies, was designed to make childcare easier for his wife and encourage the baby to be more mobile and healthy. However,  it was misinterpreted as a cruel device to raise children in a box, which caused a backlash. Despite this, Skinner is still regularly voted as the most influential psychologist of the 20th century, and radical behaviourism still exists in ideas such as the controversial use of Applied Behavioural Analysis, or “behavioural engineering”, on people with autism.