Answer:
Modern IQ tests run off of a statistically normalized distribution. 100 is normal intelligence based on the average intelligence of the group. It also uses a spread of 15 per standard deviation (e.g. if someone is 2 standard deviations above the average, they have an IQ of 100 + 15*2=130; similarly, if someone is 2 standard deviations below the average, they have an IQ of 100 - 15*2=70).
