Final answer:
George Miller is the psychologist who proposed the 'magic number 7' theory in 1956, indicating that the average short-term memory capacity is approximately 7 items plus or minus 2.
Step-by-step explanation:
The individual who originally proposed the theory regarding short-term memory capacity known as the 'magic number 7' is George Miller. In a seminal paper published in 1956, Miller summarized research suggesting that the average person can retain between 5 and 9 information items in their short-term memory, which he famously described as a capacity of 7 plus or minus 2. However, it's worth noting that more recent research by Cowan in 2010 revised this estimate to 4 plus or minus 1 for the working memory capacity.