Blink: The Power of Thinking Without Thinking (2005) is Canadian writer Malcolm Gladwell's second book. It presents in popular science format research from psychology and behavioral economics on the adaptive unconscious: mental processes that work rapidly and automatically from relatively little information. It considers both the strengths of the adaptive unconscious, for instance in expert judgment, and its pitfalls, such as prejudice and stereotypes.

The author describes the main subject of his book as "thin-slicing": our ability to use limited information from a very narrow period of experience to come to a conclusion. This idea suggests that spontaneous decisions are often as good as—or even better than—carefully planned and considered ones. To reinforce his ideas, Gladwell draws from a wide range of examples from science and medicine (including malpractice suits), sales and advertising, gambling, speed dating (and predicting divorce), tennis, military war games, and movies and popular music. Gladwell also uses many examples of regular people's experiences with "thin-slicing," including our instinctive ability to read minds, which is how we can get to know a person's emotions just by looking at his or her face.

Gladwell explains how an expert's ability to "thin slice" can be corrupted by their likes and dislikes, prejudices, and stereotypes (even unconscious ones). A particular form of unconscious bias Gladwell discusses is psychological priming. He also discusses the implicit-association test, designed to measure the strength of a person's subconscious associations/bias.

Gladwell also mentions that sometimes having too much information can interfere with the accuracy of a judgment, or a doctor's diagnosis. In what Gladwell contends is an age of information overload, he finds that experts often make better decisions with snap judgments than they do with volumes of analysis. This is commonly called "analysis paralysis." The challenge is to sift through and focus on only the most critical information. The other information may be irrelevant and confusing. Collecting more information, in most cases, may reinforce our judgment but does not help make it more accurate. Gladwell explains that better judgments can be executed from simplicity and frugality of information. If the big picture is clear enough to decide, then decide from this without using a magnifying glass.

The book argues that intuitive judgment is developed by experience, training, and knowledge. For example, Gladwell claims that prejudice can operate at an intuitive unconscious level, even in individuals whose conscious attitudes are not prejudiced. One example is the halo effect, where a person having a salient positive quality is thought to be superior in other, unrelated respects. The example used in the book is Warren G. Harding. Henry Daugherty was impressed by Harding's appearance of respectability, and helped him become president of the United States of America, though Harding did nothing extraordinary for his political career.

Gladwell uses the 1999 killing of Amadou Diallo, where four New York policemen shot an innocent man on his doorstep 19 times, as another example of how rapid, intuitive judgment can have disastrous effects.

The book begins with the story of the Getty kouros, which was a statue brought to the J. Paul Getty Museum in California. It was thought by many experts to be legitimate, but when others first looked at it, their initial responses were skeptical. For example, George Despinis, head of the Acropolis Museum in Athens, said "Anyone who has ever seen a sculpture coming out of the ground could tell that that thing has never been in the ground". Gradually, the argument for the legitimacy of the kouros' provenance fell apart. The letters tracing its history turned out to be fakes, referencing postal codes and bank accounts that did not exist until after the letters were supposedly written. However, experts to this day are unsure whether the kouros is authentic or not. The museum notes that "anomalies of the Getty kouros may be due more to our limited knowledge of Greek sculpture in this period rather than to mistakes on the part of a forger."

John Gottman is a researcher on marital relationships whose work is explored in Blink. After analyzing a normal conversation between a husband and wife for an hour, Gottman can predict whether that couple will be married in 15 years with 95% accuracy. If he analyzes them for 15 minutes, his accuracy is around 90%. But if he analyses them for only three minutes, he can still predict with high accuracy who will get divorced and who will make it. This is one example of when "thin slicing" works.