Dorothea Dix's approach to reforming mental health care was characterized by her unwavering commitment to the humane treatment of the mentally ill and the establishment of specialized care facilities. Her key aspects included:

• Exhaustive Research: Dix conducted extensive research into the conditions of asylums and the treatment of the mentally ill, documenting her findings in detailed reports.
• Advocacy and Lobbying: She used her research to lobby state legislatures and the United States Congress, presenting detailed and compelling cases for reform.
• Establishment of Hospitals: Her efforts led to the creation of new mental health hospitals and the improvement of existing ones, focusing on providing proper care and treatment rather than mere confinement.
• Education and Awareness: Dix worked tirelessly to educate the public and lawmakers about the needs of the mentally ill, dispelling common misconceptions and prejudices.
• International Impact: Her reform work extended beyond the United States, influencing mental health care in Europe and Canada.
• Through these methods, Dix fundamentally changed the way society viewed and treated mental illness, shifting from neglect and abuse to a more compassionate and medical approach.

G. Stanley Hall's views had a significant impact on the field of developmental psychology. He is often credited with founding this field, and his work laid the foundation for future research and theories in child development. Hall's approach was heavily influenced by evolutionary theory; he viewed child development as a process of evolution and adaptation. He believed that understanding the stages of child and adolescent development was crucial for comprehending human psychology as a whole. Hall introduced the concept of "adolescence" as a distinct developmental stage, characterizing it as a period of storm and stress. This perspective led to a greater emphasis on studying the developmental changes that occur throughout a person's life, particularly during childhood and adolescence. Hall's ideas also influenced education, advocating for educational systems that were sensitive to the developmental stages of children. His advocacy for age-appropriate education and the establishment of child study as a scientific field are among his lasting contributions to developmental psychology.

Charles Darwin's theory of natural selection had a profound influence on psychological thought, especially in the formation of evolutionary psychology. His theory suggested that behaviors, like physical traits, are subject to the process of natural selection. This meant that certain psychological traits and behaviors could be understood as adaptations that have evolved over generations to increase an organism's chances of survival and reproduction. This perspective encouraged psychologists to explore the evolutionary origins of behaviors and mental processes, leading to explanations for complex human behaviors such as altruism, mating strategies, and even mental disorders from an evolutionary standpoint. For example, fears and phobias can be seen as evolutionary adaptations to avoid dangerous situations. Darwin's ideas also influenced the study of animal behavior, leading to comparative psychology, which compares the psychological processes of humans and other animals. Overall, the incorporation of evolutionary theory into psychology provided a new lens for understanding the biological basis and evolutionary history of human and animal behavior.

Mary Whiton Calkins' work in self-psychology was groundbreaking and distinguished significantly from other psychological approaches of her time. Unlike her contemporaries who often focused on isolating and analyzing individual elements of consciousness (as seen in structuralism), Calkins proposed a holistic approach. She emphasized the unity and continuity of the conscious experience. Her approach considered the self as a singular entity, which was contrary to the then-popular belief of breaking down mental processes into discrete parts. Calkins' theory underscored the importance of personal identity and the interconnectedness of mental processes in understanding consciousness. This perspective was a precursor to later humanistic approaches in psychology, which similarly emphasized the holistic nature of human experience and the importance of personal growth and self-actualization. Her work, therefore, represented a significant departure from the reductionist tendencies of early psychology, advocating for a more integrative view of the mind.

Sigmund Freud's psychoanalytic theory, while revolutionary, faced several major criticisms that have influenced its perception in modern psychology. One of the primary critiques was the lack of scientific evidence supporting his concepts. Many of Freud's theories, such as the Oedipus complex and the structure of the psyche, were based on introspective methods and case studies rather than empirical research, leading to questions about their generalizability and scientific validity. Additionally, Freud's emphasis on sexuality as a driving force in human behavior was controversial and considered overemphasized by many contemporary psychologists. His theories were also criticized for being deterministic, suggesting that early childhood experiences irreversibly determine an individual's psychological development. In modern psychology, while Freud's theories are recognized for their historical importance and for introducing the concept of the unconscious mind, they are often viewed with skepticism regarding their scientific basis. However, his influence is still seen in areas such as psychodynamic therapy, the exploration of defense mechanisms, and the recognition of the complexity of the human psyche.