Primacy effect is the tendency to over-weight the information encountered first. Like recency, it is a serial-position phenomenon — Solomon Asch’s classic impression-formation studies showed that the same list of traits produced a markedly more favorable judgment when the positive words came first than when they came last. The opening does not just get remembered; it sets a frame that later information is read through, so early input shapes the interpretation of everything that follows.
Manipulation exploits this by seizing the first move: a loaded headline before the article, an opening characterization before the evidence, a label attached to a person or option before you can assess it yourself. Once the frame is set, the ordinary machinery of confirmation bias (T8.1) does the rest — ambiguous facts are resolved in the frame’s favor, and contradicting facts are discounted or explained away. This is why reputation attacks and pitches both fight so hard to speak first: the opening buys interpretive control that later correction struggles to reclaim.
Defense is a discipline of suspended judgment. Treat the opening as one data point rather than the conclusion, and refuse to render a verdict until the full picture is in. Re-ordering the evidence — considering it in a different sequence, or deliberately hunting for the counter-case — strips the first frame of its privilege. An honest case survives being examined out of order; a primacy play depends on you never revisiting the frame it set.