Determinism is the approach that all behaviour is predictable and caused by stimulus in our environment that we cannot control. B.F Skinner believed that free will was an illusion and that our choices are determined by our environment. He denied the existence of specific internal psychologies such as goals, purposes, aims, intentions etc. He was convinced that science can determine predictable behaviour in his book “Beyond Freedom and Dignity”.

After taking a closer look, Skinner was not far off. We can see evidence of this in various practical settings whether backed by science or not. When looking at identical twins raised in different environments we can see clear differences in behaviour. There are very few similarities except for genetic similarities. (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4563825/)

On the contrary, the "Jim twins'' show an opposing idea. Identical twins separated at birth and raised in different environments reunited after 39 years and somehow led a very similar lifestyle. The Minnesota twin study shows evidence that genetics play a role in behavioural development as well. This could also be seen as internal determinism in a way, where their biological state can explain some behavioural aspects like neuroticism or extroversion. External determinism is the external factors such as parents and environment can affect the overall behaviour.

John B. Watson, a well rounded behaviourist conducted a conditioning experiment on a child. The experiment was called Little Ablert. A 9-month old child was exposed to various stimuli that he did not fear at first and then introduced to a white rat in combination with a loud noise. The child began to cry after hearing the loud noise. Afterwards whenever introduced to a white rat, Albert began to cry. Even when introduced to a white object or a furry object. This is a classic practical example of conditioning due to the environmental stimuli. (https://www.verywellmind.com/the-little-albert-experiment-2794994)

References:

Bahjat, Mudhaffar, "“Sources of Human Psychological Differences: The Minnesota Study of Twins Reared Apart” (1990), by Thomas J. Bouchard Jr, David T. Lykken, Matthew McGue, Nancy L. Segal and Auke Tellegen". Embryo Project Encyclopedia (2017-10-19). ISSN: 1940-5030 http://embryo.asu.edu/handle/10776/12997.

McLeod, S. A. (2019, April 11). Freewill vs determinism. Simply Psychology. https://www.simplypsychology.org/freewill-determinism.html