Prevalence rates for obesity are on the rise in the United States and in other parts of the developed world where lifestyles make it easy for people to take in more calories than their bodies use. How do people take in too many calories?

Obsessive-Compulsive Disorder

Many people knock on wood to ward off bad luck. Others may walk around, rather than under, ladders, or they may step over, rather than on, cracks in the sidewalk.

Oppositional Defiant Disorder

*oppositional (op-po-ZI-shun-al) is an attitude of going against something or refusing in a combative way.


Carla was riding her bicycle to school when a speeding car ran a stop sign. The driver slammed on the brakes, but the car kept skidding toward Carla with a sickening squeal.


One day John came across a group of his friends huddled together on the soccer field. As he approached, they were all talking and laughing and enjoying themselves.

Peer Pressure

A person's friends may dress a certain way, comb their hair in a particular style, and have certain ideas about music and movies. Some teenagers may not share these opinions or adopt these fashions, but they may feel that they should.


Dan is a warmhearted, energetic man who knows everyone in the neighborhood. He is always ready to lend a hand or a tool from his well-stocked garage.

Personality Disorders

Personality, or personality style, is someone's usual pattern of thinking, feeling, and behaving. Personality style is made up of a number of personality traits or characteristics.

Pervasive Developmental Disorders

Pervasive developmental disorders are conditions that prevent children from developing normal communication and normal social abilities. Signs of these conditions begin to appear in the very early years of childhood.


Sigmund Freud (1856-1939), one of the greatest psychological thinkers in history, was a psychiatrist and the founder of psychoanalysis (SY-koa-NAL-i-sis), a treatment method that focuses on deeply hidden fears and internal conflicts. One of Freud's most famous cases involved Little Hans, a 5-year-old boy who had a great fear of white horses with black mouths.

Post-Traumatic Stress Disorder

Jeff had never belonged to a gang, but a year ago, he was caught in the crossfire of gang violence. As he walked down the street, Jeff was shot in the chest by a boy who mistook him for a member of a rival gang.


"Psychosis" is a medical term used to describe serious mental disorders that cause a person to lose touch with reality. People with psychosis may have delusions*, hallucinations*, or dementia*; they may lose the ability to speak coherently or to understand what others say to them; and their thoughts, feelings, and behaviors may be inappropriate and disconnected from the reality around them without their being aware of the disconnection.


Rape is forced, unwanted sexual intercourse involving the genitals or any other part of the body. Rape also is called sexual assault.


A hot, bubbly bath may ease stress for Shanna, while an on-your-toes racquet ball game releases the day's tensions for Wayne. Maria finds that hot cocoa with a friend is "pure relaxation" and the antidote to stress.


Abraham Lincoln experienced a number of failures that could have left him feeling sorely defeated. He failed in business as a storekeeper.

Rett Disorder

A girl with Rett disorder develops and acts normally for some time after she is born. At first she seems to be perfectly healthy.


Information in the form of electrical signals flows down nerve cells in the brain, triggering the release of neurotransmitters. These chemical messengers transmit information from one nerve cell to another.

School Avoidance

Ben missed a lot of school because of his stomachaches. His stomach felt especially bad on Monday mornings.

School Failure

School failure is a process where a student slips farther and farther behind his peers and gradually disconnects from the educational system. The end result of school failure is dropping out before graduation.

Seasonal Affective Disorder

Many people get the winter blahs or cabin fever as the days get shorter and colder, but for about 10 million Americans with SAD, shorter days mean a slide into true depression. SAD is a seasonal pattern of depression.