How does scientific consensus form, and what does it actually mean?
Through the accumulation of independent, mutually reinforcing evidence across many researchers and methods, until a position becomes the working assumption of a field — and it is a description of the state of evidence rather than a vote or an authority.
How it actually develops:
Multiple independent lines of evidence converge. This is the crucial mechanism. A conclusion supported by one method can be an artefact of that method; a conclusion supported by several unrelated methods, each with different weaknesses, is far harder to explain away. Consilience is the term for this convergence, and it does more work than any single study.
Replication and extension, with findings holding in different populations and conditions.
Mechanistic explanation — not merely that something happens but why, consistent with established understanding.
Failure of alternatives. Competing explanations are tested and fail to account for the evidence.
Synthesis through reviews and assessment bodies, which is where a scattered literature becomes a stated position.
What consensus is not:
Not unanimity. There are almost always dissenting researchers, and their existence is not evidence that the consensus is weak.
Not a vote. Counting scientists is a proxy for the state of evidence, not the thing itself.
Not permanent. Consensus positions have been overturned, which is how the process is supposed to work.
Not an argument in itself. The consensus is a summary of the reasons; it is not a substitute for them.
Why it is nonetheless reasonable for non-specialists to rely on. Nobody can evaluate the primary evidence in every field. The alternative to relying on expert consensus is not independent judgement but selecting whichever claim appeals — which is systematically worse. The relevant question is whether the consensus-forming process in that field is functioning, not whether you personally find the conclusion intuitive.
How consensus is misrepresented: citing a small number of dissenters as though the field were divided — false balance; pointing to historical overturned consensus as though that made all consensus unreliable; and manufacturing apparent disagreement, a documented strategy in several industries.
Genuine disagreement is visible in the technical literature, with reasons given, rather than in public campaigns.