It should be stated plainly what astrologers claim and what the testing record shows, because the two are often conflated by both defenders and critics.
The claim in most contemporary Western practice is not a claim of physical causation. Very few working astrologers hold that Saturn emits an influence. The usual formulations are either correspondence, that celestial and terrestrial patterns run in parallel without one causing the other, or a frankly symbolic and psychological reading, in which the chart is treated as a language for describing character and timing rather than as a mechanism. The stronger claim, that a chart supports specific prediction of events, is made mainly in the horary and electional branches and in traditional revival circles.
Formal testing has addressed the strong version, and the results are consistently negative. The best-known controlled trial, published in a leading science journal in 1985, asked astrologers to match natal charts to personality profiles under double-blind conditions and found performance at chance. Large-scale record studies, of time twins, of birth dates against occupation and against personality inventories, have likewise failed to find effects. The Gauquelin work of the mid-twentieth century, which reported an association between planetary positions at birth and eminence in certain professions, is the one body of results that generated sustained scientific argument; attempted replications and the dispute over selection effects left it unsettled and it is no longer treated as supporting evidence by most researchers.
Astrologers answer along several lines: that the chart is not the sort of thing a personality inventory can test, that interpretation depends on skill that a matching task removes, and that the practice is hermeneutic rather than predictive. Those are coherent replies. They are also, in the strongest versions, replies that place the claim outside the reach of testing altogether, and that consequence is worth naming.