Definitions of Abnormality

1. Definitions of Abnormality • Defining a person or behaviour as ‘abnormal’ implies something undesirable and requiring change • Therefore, we must be careful how we use the term • Psychologists need methods for distinguishing ‘normal’ from ‘abnormal’

2. Definitions of Abnormality • Our definition of abnormality must be objective: • It must not depend on anyone’s opinion or point of view • It should produce the same results whoever applies it • It must not be under- or over-inclusive • It must not label as ‘abnormal’ or ‘normal’ behaviours or traits that aren’t

3. Definitions of Abnormality • There are various definitions of abnormality • Deviation from… • statistical • social norms • Personal Distress • Dysfunction • Danger to self and/or others

4. Statistical Infrequency • Under this definition, a person’s trait, thinking or behaviour is classified as abnormal if it is rare or statistically unusual. • With this definition it is necessary to be clear about how rare a trait or behaviour needs to be before we class it as abnormal

5. Normal distribution • We can work out whether a person’s thinking/behaviour is statistically abnormal by using standardized tests which gather quantitative data • Most psychological variables have what is known as normal distribution • This means that if you take a large enough random sample, you will find that when you plot the scores for most psychological variables, you will end up with a bell curve or normal distribution curve • Roughly 50% of people score above the mean and 50% score below the mean

6. So, how do we work out who is abnormal? • Having calculated the mean (a measure of central tendency), we next need to work out the standard deviation, a measure of dispersion which tells us on average how much the scores in our sample differed from the mean • Most people, (approx 95%) score between one and two standard deviations above or below the mean • This leaves just over 5% of people who score unusually high or low (more than or less than 2SDs from the mean) and these people would be classed as abnormal because they are not typical of most people

7. A very small subset of the population (<2.2%) have an IQ below 70pts. Such people are statistically rare. We regard them as having abnormally low IQs Statistical Infrequency frequency 70 100 130 IQ Scores

8. Z scores: exactly how abnormal am I? • Statisticians use Z scores to indicate how many standard deviations an individual’s score lies above or below the mean for any specific psychological variable • for example ... • a z score of +1 would suggest that the person had a score which was above the mean but within one standard deviation • a z score of +2 would mean they were above the mean but within 2 standard deviations • a z score of +3 would mean they had an abnormally high score which was in excess of 2 SDs above the mean • Likewise... • a z score of -1 would suggest that the person had a score that below the mean but within one SD • a z score of -2 would mean they scored upto 2SDs below the mean • a z score of -3 would mean they had an abnormally low score which was in excess of 2 SDs below the mean