Rats are more likely to engage in intracranial self-stimulation if they also self-administer a drug that further increases the sensitivity of the pleasure pathways in the brain. This drug is

For which of the following cross-sectional research findings would you be MOST likely to suspect a "cohort effect" as the primary explanation? a. lower bone density among 70-year-olds than among 35-year-olds

b. faster recall of lists of words by 20-year-olds than by 60-year-olds c. higher percentage of blue-collar workers among 50-year-olds than among 30-year-olds d. a lower rate of marital satisfaction among couples in their 30s than among couples in their 50s

Which of the following are withdrawal symptoms from caffeine?

a) headaches b) anxiety c) fatigue d) all of the above