What Is the Similarity Illusion and Why Does It Matter?

The similarity illusion is a cognitive bias where we assume others share our beliefs, values, and habits more than they do. This mental shortcut can lead to surprise when we discover a good friend, whose tastes we thought we knew, dislikes a favorite movie or has different political views. It operates without our conscious awareness, shaping our expectations of those around us and influencing how we form connections.

Psychological Drivers of Assumed Similarity

The tendency to assume similarity is a mental shortcut, or heuristic. Our own mind serves as the most readily available reference point, so we use our own thoughts and feelings as a baseline for what others are thinking. This process is a form of social projection, where we cast our internal state onto others, allowing the brain to make social judgments quickly.

This bias is also driven by motivational factors. Assuming that others see the world the way we do can bolster self-esteem by validating our own choices and perspectives. If others share our beliefs, it reinforces the idea that our own way of thinking is correct. This fulfills a deep-seated need for social connection, making others seem more familiar, predictable, and approachable.

How the Illusion Shapes Personal Relationships

Within close personal relationships, the assumption of similarity can create significant friction and misunderstanding. In friendships or romantic partnerships, one person might incorrectly believe their partner shares their emotional reactions. For instance, someone might make a sarcastic comment, assuming their partner will find it funny because they would, only to be surprised when their partner is genuinely hurt.

This cognitive bias often leads to poorly conceived gestures that fall flat. A person might plan a surprise party or a vacation based entirely on their own desires, projecting their preferences onto their partner without considering their partner’s tastes. They assume that what brings them joy will automatically bring their partner joy, leading to disappointment when the reaction is not what they expected.

These miscalculations stem from a failure to recognize the other person as a separate individual with a unique internal world. The similarity illusion creates a blind spot where we substitute our own feelings and desires for those of our loved ones. Over time, these repeated misunderstandings can erode intimacy and trust, as one or both partners may feel unseen or consistently misread by the person who is supposed to know them best.

Consequences in Group Settings

When scaled up to group settings, the similarity illusion plays a powerful role in shaping social dynamics. It fosters in-group cohesion by creating an unspoken assumption that “we all think alike.” This sense of shared reality can strengthen bonds within a team, community, or social group, making cooperation feel effortless and natural.

This same mechanism, however, can distort perceptions of those outside the group. The illusion can lead to a failure to see the diversity of thought within out-groups, promoting stereotypes. It becomes easier to view an opposing political party or a rival company as a monolithic entity where everyone holds the same extreme views. This “us versus them” mentality is amplified by the assumption that the other side is uniformly different from one’s own group.

In a workplace environment, this bias can manifest as groupthink, where a team assumes universal agreement on a decision. Dissenting opinions may be unconsciously suppressed because everyone presumes consensus, stifling creativity and critical analysis. In the political sphere, it fuels polarization by making the views of the “other side” seem not just different, but incomprehensibly alien, widening the gap between different ideological groups.