Does Your Star Sign Affect Your Personality?

Astrology is one of the oldest and most specific personality claims still in wide circulation: that the position of the sun at your birth sorts you into one of twelve categories with a predictable temperament. It is also one of the rare personality claims that has actually been put to a proper blind test - more than once, at a large scale - and it did not survive the test.

A test astrologers helped design

In 1985, the physicist Shawn Carlson published a study in the journal Nature that did something unusual: he had professional astrologers help design the test of their own claims, so no one could later object that the method had been unfair. Astrologers were given a person's full natal chart and three personality profiles, one of which genuinely belonged to that person (based on a well-established personality inventory, the California Psychological Inventory), and were asked to pick which profile matched the chart. In a separate part of the study, the people themselves were given their own astrologically-generated chart interpretation, alongside two others, and asked to pick out their own. Across both tests, the astrologers - and the sitters themselves - matched charts to profiles at a rate no better than chance would predict. Carlson's own conclusion, printed in the paper, was that the results "strongly refute the astrological thesis".

Why the blind part mattered

The double-blind design is what makes Carlson's result hard to dismiss. If astrologers had simply been asked whether their own predictions felt accurate, the Barnum-style effect described below would have made almost any chart feel like a decent fit. By forcing a choice between genuinely different profiles, with neither the astrologer nor, in the second test, the sitter knowing in advance which profile was the correct one, the study removed the room for that kind of self-flattering interpretation and left only the raw ability to match a chart to a person - which is precisely the ability astrology claims to have.

A very large second look

A single test can always be dismissed as unlucky or unrepresentative, so it matters that later research checked the same basic idea at a much larger scale. In 2006, the psychologists Peter Hartmann, Martin Reuter and Helmuth Nyborg combined two large, pre-existing datasets - one of more than 4,000 middle-aged men from a long-running study of Vietnam-era US veterans, and another of more than 11,000 young adults from a US national survey - and checked whether date of birth predicted personality or general intelligence in any way that matched astrological sun signs. With more than 15,000 people and a wide range of personality and cognitive measures, the study had the statistical power to detect even a small real effect if one existed. It found none. Their own summary was direct: the study "provides no evidence for the existence of relevant relationships between date of birth and individual differences in personality and general intelligence".

So why does a horoscope feel so accurate?

None of this explains why so many people read a description of their sign and feel genuinely seen. That gap has its own well-documented explanation, and it has nothing to do with the stars. In 1949, the psychologist Bertram Forer gave a classroom of students a personality description assembled from vague, generically flattering lines lifted from an astrology book, told each of them it was written specifically for them, and asked them to rate its accuracy. The class rated it as a strikingly good match - even though every student had received the identical text. Statements built to apply to almost everyone ("you have a need for other people to like and admire you") get read as personal insight because the reader supplies the specific memories that seem to confirm them. This pattern, since nicknamed after the effect it produces, works on horoscope descriptions exactly as it worked on Forer's fake profile: broad, agreeable statements feel accurate regardless of what actually generated them.

What is left standing

It would be a mistake to conclude from this that no personality description can be trustworthy, and that is really the more useful lesson here. The Big Five model of personality - openness, conscientiousness, extraversion, agreeableness and emotional stability - has been tested the same way astrology was: checked against independent measures, retested over time, and checked for whether it predicts anything outside itself, and it consistently holds up in ways that sun-sign astrology does not. The difference is not that one is "science" and the other "belief" in some abstract sense; it is that one has been tested against reality and kept producing the same answer, and the other has been tested and kept failing.

What this means for you

None of this is a reason to feel embarrassed about enjoying horoscopes, reading about your sign, or finding a certain amount of fun in the ritual - plenty of people who know the evidence still do. The honest position is simply to hold the two activities apart: astrology as entertainment and a prompt for self-reflection is fine; astrology as a validated account of your personality is not supported by the tests it has actually been given. If you are curious how a description of you holds up when it is checked properly, our Big Five test uses the model with the strongest evidence behind it, our which element test is a lighter, for-fun alternative to sun signs that we label as such, and our paranormal beliefs test looks at how people relate to claims like this one more generally.