Illustration according to the research of Hans Jürgen Eysenck in 1967
Extraverts need more stimulation to get aroused and therefore they seek more stimulation. Introverts are viewed to be more responsive and physiologically affected by arousing stimuli than extraverts and reach their peak of excitement earlier than extraverts. Once they reach this peak they tend to back out to prevent over-excitement. Introverts do better in low stress situations, extraverts in high stress situations.
In direct marketing and advertising a number between 6 and 13 contacts has been suggested to be able to generate new leads and to have a positive influence on purchase decisions. Looking at the graph it becomes clear that there needs to be a differentiation between extravert and introvert customers.
A number of 13 contacts would be far too high for introverts. Such offensive direct mail would be marked as spam. Once an email address ends up on the block list, chances are close to zero that the next email sent from this address will find its way into the inbox. Thus the company has lost its connection to a potential customer, simply because it ignores the customer's need for privacy.