humanflow

Burstiness

Burstiness is how much sentence length and complexity vary across a passage — human writing tends to alternate between long sentences and short ones, while generated text tends to settle into a consistent rhythm.

Last reviewed 15 August 2026 · The HumanFlow team

In plain English

Read a paragraph out loud and listen to the shape of it. If the sentences all take about the same breath, burstiness is low. If a nine-word sentence lands after a thirty-word one, burstiness is high.

The term comes from the observation that human attention is bursty: a writer labours over a difficult idea, then dispatches the next point in six words because it is obvious.

A worked example

The same argument, written twice. Sentence lengths are counted after each one so the pattern is visible rather than asserted.

A. Social media has changed how people communicate with each other. (10)
   These changes have been documented by many researchers over time. (10)
   The effects appear to be both positive and negative in nature. (11)

B. Social media rewired how we talk. (6)
   Researchers keep finding the same split — teenagers report more connection and more loneliness in the same survey, sometimes in adjacent questions. (22)
   That contradiction is the interesting part. (6)

A runs 10, 10, 11. The variation is one word. Nothing is wrong with any individual sentence and the paragraph is unreadable in a way that is hard to point at.

B runs 6, 22, 6. The long sentence carries the evidence; the short ones carry the claim and the judgement. That is burstiness, and it is a by-product of the sentences doing different jobs.

The useful part for a writer: you do not produce B by varying sentence length. You produce it by having a claim, evidence and a judgement, and letting each take the room it needs.

Why it matters for AI detection

Burstiness is the signal most directly under a writer's control, which makes it the most practically useful of the detection mechanics.

It is also the one that best survives being taken seriously. Rewriting for genuine variation improves the prose for a human reader, unlike most things done to move a detection score.

The caution is the same one that applies everywhere here: low burstiness is a property of formulaic writing, and plenty of legitimate formats — methods sections, legal drafting, technical documentation — are formulaic by convention and by requirement.

Commonly confused with

Perplexity
Burstiness is measured across sentences; perplexity is measured word by word. A passage of uniformly-lengthed but highly unusual sentences is low burstiness and high perplexity.

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Part of the AI detection glossary.