Assumed similarity is the tendency to believe other people think, feel, and behave the way you do. When you meet someone new, your brain uses what it knows best (yourself) as a template for predicting what that person is like. This mental shortcut is sometimes called the “assumed similarity bias” or the “self-based heuristic,” and it shapes everything from first impressions to hiring decisions to romantic attraction.
How the Bias Works
When you have limited information about someone, your mind fills in the gaps using your own personality, preferences, and attitudes as a reference point. If you’re an optimistic person, you’re more likely to assume a stranger is also optimistic. If you value punctuality, you might expect others do too, and feel genuinely surprised or annoyed when they don’t.
What makes this bias distinct from a general tendency to see people positively or negatively is that it’s anchored to your specific traits. Researchers define “distinctive assumed similarity” as the tendency to project onto others the traits that make you different from most people. It’s not just assuming everyone is average. It’s assuming everyone shares the qualities that make you, specifically, who you are. So a person who is unusually introverted doesn’t just assume others are somewhat quiet; they overestimate how introverted other people are in ways that go beyond what’s typical.
Perceived Similarity vs. Actual Similarity
One of the most striking findings about assumed similarity is that the similarity you perceive matters far more than actual, measurable similarity, at least when it comes to liking people. A series of speed-dating studies found that perceived similarity strongly predicted romantic attraction, while actual similarity (measured by comparing participants’ real personality traits and preferences) did not predict attraction at all.
The numbers are stark. When researchers meta-analyzed 26 measures of actual similarity between speed-dating partners, the average effect on romantic liking was essentially zero. But when participants simply felt similar to their partner, the association with liking was enormous. General perceived similarity predicted romantic liking with a remarkably strong relationship, far outpacing any effect of objectively shared traits.
This means the story you tell yourself about how alike you are to someone carries real weight in your social life. Two people can be genuinely different in personality and values, yet feel a strong connection because each one projects their own qualities onto the other. Conversely, two people who are objectively very similar may never click if neither perceives the overlap.
Ingroups Get More of It
Assumed similarity doesn’t apply evenly to everyone you meet. People consistently assume greater similarity with members of their own group than with outsiders. If someone shares your nationality, your profession, your religion, or even just your favorite sports team, you’re more likely to assume they also share your opinions, habits, and personality traits.
This has a cascading effect. Because perceived similarity drives liking, and because you assume more similarity with ingroup members, you’re predisposed to feel warmer toward people in your own group and cooler toward people outside it. The bias reinforces social boundaries that may have little basis in how similar people actually are.
First Impressions and Low Information
Assumed similarity is strongest when you know the least about someone. During a brief first meeting, you have almost no real data about the other person’s inner life, so your brain leans heavily on the self-based heuristic to make predictions. As you spend more time with someone and gather actual information about their preferences and personality, the bias typically weakens because real data replaces the guesswork.
This is why first impressions can be so misleading. You may walk away from a short conversation feeling a strong sense of connection, not because the other person genuinely shares your values, but because you unconsciously projected those values onto them. The feeling of rapport was real; the similarity behind it may not have been.
Effects on Hiring and Workplace Decisions
Assumed similarity creates a specific risk in professional settings, particularly during unstructured job interviews. When an interviewer and a candidate share a background, hobby, or personality style, the interviewer may unconsciously rate the candidate more favorably. Giving preference to a candidate because they like the same sport as you is relatively harmless if the candidate also meets the job criteria. Favoring an introverted candidate because you’re introverted, when the role requires constant social interaction, is a different problem entirely.
The good news is that the bias can be managed. Large-scale studies of real (not simulated) interviews found that trained professionals using structured interview methods showed little or no similarity effect in their hiring decisions. When recruiters had access to standardized personality assessments alongside interviews, they focused on how well the candidate fit the role rather than how much the candidate reminded them of themselves. Structure and objectivity are the most reliable antidotes.
Why It Sometimes Looks Like Accuracy
Assumed similarity isn’t always wrong. If you happen to be similar to someone, then projecting your own traits onto them produces a correct impression. Researchers are careful to separate these cases from genuine bias by controlling for actual similarity between two people. Only after removing the overlap that really exists can you see the remaining projection, the part where you’re seeing yourself in someone who isn’t actually like you.
This distinction matters because it explains why the bias feels so trustworthy. Every time your assumption of similarity turns out to be correct (because you genuinely were similar to that person), it reinforces your confidence in the heuristic. The times it fails are less memorable, so the pattern persists unchecked. Recognizing that some of your “accurate reads” on people are actually lucky projections is the first step toward forming more realistic impressions of the people around you.