Although psychology has many subdisciplines, the field shares many themes. These include reliance on the scientific method, recognizing that nature and nurture interact to shape outcomes, and acknowledging individual differences.
The scientific method applies to all branches of psychology. No matter what question psychologists ask, they take an empirical approach, basing their understanding of the world on measured and observed phenomena rather than beliefs. Each researcher builds on the prior work of other researchers. For example, developmental psychologist Mary Ainsworth (1913–99) is known for her work on mother-infant attachment. To investigate toddler attachment to parents, she developed a task called the "Strange Situation." Her team observed toddlers in various situations: with their mother, alone with a stranger, with their mother and the stranger, and alone in a room. Children reacted in various ways. Some were inconsolable even after their parent returned, some were easily soothed, and others avoided their parent despite a clear desire for connection. Based on her observations, Ainsworth developed a model of attachment styles. She then conducted other studies to test this model. Other researchers expanded upon her work to show how childhood attachment shapes adult relationships.
Ainsworth's work exemplifies the evolution of psychology as a discipline. Like psychodynamic theorists, she acknowledged the importance of early childhood experiences in individual development. Like behaviorists, she tested her hypotheses by directly observing children's behavior. Like cognitive psychologists, she acknowledged the role of thoughts and emotions in human behavior.
All areas of psychology recognize that mental states and behavior are multiply determined. Both nature and nurture play a role in shaping outcomes. For example, two children with a biological predisposition to be disorganized and inattentive could have very different outcomes. A disorganized child taught good study skills could get good grades and develop strong self-esteem. A child without that support may feel overwhelmed and incompetent, ending up with poor grades and poor self-esteem.
The biopsychosocial model considers the complex interactions between biological, psychological, and social factors in understanding behavior and mental states. Research and clinical work are grounded in the understanding that no one cause of behavior should be considered in isolation. For example, an anxiety disorder is not caused by any one life event or aspect of biology. Instead, anxiety emerges from a combination of biological risk factors, psychological tendencies, environmental stressors, learning history, and life events.
Another commonality across areas of psychology is the recognition of individual differences. Psychologists acknowledge that no two people are exactly the same. Even identical twins raised together have differences in personality traits, skills, and behaviors. It is often those differences that inspire psychologists to study the complexities of human behavior. Recognition of individual differences is particularly important for diagnosing and treating psychological disorders. In any group of people diagnosed with depression, there will be significant differences in causes, symptoms, and optimal treatment approaches.