Within a fraction of a second of meeting someone new, most people have already formed an impression. They have decided, often without realising it, whether the stranger is friendly or hostile, confident or anxious, trustworthy or not. These judgements feel unavoidable, and in many everyday situations they are. The question that interests psychologists is not whether first impressions exist but how accurate they are, how stable they prove to be, and how much damage they can do when they are wrong.

Studies of rapid social judgement date back to the 1970s, when the American psychologist Nalini Ambady introduced the idea of 'thin slices': very short extracts of behaviour, sometimes as brief as six seconds, used as stimuli in experiments. Volunteers were asked to rate the people they saw on qualities such as warmth, competence or honesty, without any background information. Their ratings were then compared with more detailed measurements taken over longer periods, such as student evaluations of teachers gathered at the end of a semester. The striking finding was that the thin-slice judgements correlated surprisingly well with the longer-term measures, even when the extracts contained no sound.

Results of this kind could easily be used to argue that first impressions are usually correct. That interpretation goes too far. A closer look at the thin-slice studies shows that they measure agreement, not truth. Volunteers watching a short clip of a teacher tend to agree with each other about how warm or competent the teacher seems, and their consensus tends to match that of students who have attended the teacher's lectures for months. But the students themselves may be forming their longer-term opinion largely on the basis of the same cues, so the two measures are not independent. The fact that first impressions are shared does not mean they are accurate.

Stability is a related issue. Once formed, first impressions are remarkably resistant to change, even when new information contradicts them. Dr. Heloise Chen, a social psychologist at the University of Melbourne, has shown experimentally that people who first meet a stranger in a stressful situation (say, during a noisy group exercise) continue to rate that person as anxious in later, calmer meetings, even when the stranger's behaviour has clearly shifted. This is sometimes called the 'halo of context': the setting of the first encounter colours every subsequent encounter for a long time.

The consequences of stable but inaccurate impressions can be serious. In hiring, interviewers who rely heavily on their initial feelings about a candidate tend to spend the rest of the interview confirming those feelings, asking questions that favour them and downplaying inconsistent responses. A 2019 review by the Centre for Evidence-Based Management concluded that unstructured interviews, in which interviewers have broad freedom to ask whatever comes to mind, are consistently worse predictors of later job performance than structured ones, in which every candidate answers the same questions in the same order. The review attributed the difference largely to the role of first impressions.

The law is another area of concern. Jurors who form an unfavourable impression of a defendant during the first minutes of a trial may interpret later evidence in a way that confirms their initial view, a pattern sometimes called 'impression anchoring'. Courts in several jurisdictions have responded by limiting what jurors see before the evidence begins. In England and Wales, for example, defendants may no longer be brought into the courtroom in visible restraints unless there is a specific reason, because the sight of chains has been shown to produce a durable impression of dangerousness that is hard to dislodge.

None of this means that first impressions should be abolished. They are often useful, and they are almost impossible to suppress entirely. What psychologists increasingly recommend is a kind of second-thought discipline: recognising that a first impression is an early guess, treating later information as evidence to be weighed rather than noise to be filtered, and designing settings - interviews, classrooms, courtrooms - so that initial cues carry less weight than they otherwise would. Dr. Chen describes this as 'slowing down the first minute', a small change in practice that, she argues, can make a disproportionately large difference to the fairness of the decisions that follow.