The most common threat to the internal validity of research designs is probably:

The smell of a sweaty person of the opposite sex has what effect on heterosexual people?

a. Increased sexual arousal for both men and women b. Increased sexual arousal for men, but not for women c. Increased sexual arousal for women, but not for men d. No increase in sexual arousal for either men or women

Which describes the normal pattern of pragmatics and mechanics intelligence development between ages 25 and 70? a. Both pragmatics and mechanics increase with age

b. Both pragmatics and mechanics decrease with age. c. Pragmatics remains constant while mechanics decreases. d. Mechanics remains constant while pragmatics decreases.