Suggestibility testing determines how you think or more so how you take in information. A very important bit if a hypnotherapist is giving you suggestions. Generally your suggestibility is determined between the ages from 0 to 8. Your primary caretaker, often your mother plays a big role in helping to determine this. A small basis of this stems from Eric Erickson's stages of development but otherwise widely popularized by Dr. John Kappas in his work in Hypnotherpy starting around 1962 and covering over 40 years.