In its 130 years of existence as a science, psychology has not arrived at a universally accepted unified theory. However, it has produced a vast number of research findings that belong to a wide variety of research areas and applications. Among them, some stand out not only for their impact on the general public but mainly for the influence they have had and continue to have on other researchers.

In his book "Forty Studies that Changed Psychology," Roger R. Hock has gathered the most significant of these landmark studies and presents them classified into ten areas of psychological knowledge, with four studies in each:
- Biology and Human Behavior,
- Perception and Consciousness,
- Learning and Conditioned Learning,
- Intelligence, Cognition, and Memory,
- Human Development,
- Emotions and Motivation,
- Personality,
- Psychopathology,
- Psychotherapy,
- Social Psychology.

The studies included in this book have been carefully selected from a multitude of studies that appear in scientific manuals or psychology journals, as well as from studies recommended by leading figures in the numerous psychological fields.

The selection of forty studies seems realistic, both from a historical perspective and in terms of the desired dimensions of the book. The studies chosen are likely the most renowned, the most important, and those that have had the greatest impact on the history of psychology. The author uses the word "likely" because some readers of the book may question certain choices.

One thing is certain: There is no single list of 40 studies that could satisfy everyone. However, the included studies remain the most frequently referenced texts by other researchers, sparked the most lively debate when published, served as a catalyst for the most subsequent related research, opened new fields of scientific inquiry, and radically changed our knowledge of human behavior.

The original studies are not included in their entirety in the book. The author addresses and summarizes them uniformly throughout the book, aiming to aid in their more complete understanding. Each reading includes the following:

1. Exact reference to the source where the original research can be found.
2. A brief introduction that succinctly describes the scientific background that led to the specific research, as well as the reasons why the respective researcher conducted it.
3. The theoretical propositions or hypotheses on which the research is based.
4. A detailed description of the experimental design and methods used in conducting the research. Where deemed necessary, details about who the participants were and how their participation was secured are included. The devices and materials used, as well as the procedures followed during the research, are also described.
5. A summary of the research results in a clear and understandable manner, without much technical-scientific and statistical language.
6. Interpretation of the findings, based on the discussion of these by the author in the original article.
7. Explanation of the importance of the study for the science of psychology.
8. Brief reference to similar or opposing findings from later research, as well as subsequent challenges or critiques from other experts in the field.
9. An indicative listing of recent applications and citations of the specific study by other articles, in order to demonstrate its ongoing impact.
10. References to other texts for reading, for additional and updated information on the research topic.

Scientists often use language that is difficult to understand (even for other scientists). The primary purpose of this book is to make the discoveries it refers to understandable and accessible to the reader and to convey some of the excitement and emotion that accompanied them.
Where Roger Hock deemed it necessary and feasible, he made certain interventions and simplifications in the described studies, always with care, so as to preserve the meaning and elegance of the original work, as well as to fully clarify the impact the research had.

Hock's book is an ideal introduction to psychology for all students who wish either to begin their studies in psychology or to learn within a semester what psychological research has managed to discover at its best moments and thus gain a broad view of the branches of psychology based on some of the most significant achievements in each branch.