Ideas change from within. Individuals can spontaneously switch traits—through innovation, error, or simply changing their mind. No copying involved.
Not all cultural change comes from transmission. Sometimes people innovate, make errors, or simply decide to do something different. This is cultural mutation.
Each time step, every individual has a small probability of switching their trait—regardless of what others have.
Unbiased mutation: equal chance of A→B or B→A. Biased mutation: change flows in one direction only.
Unbiased mutation converges to 50-50. Biased mutation drives the favoured trait to fixation with an r-shaped curve.
Unbiased: A↔B with equal probability. Converges to 0.5.
Biased: Only B→A. Trait A always wins.
When change is equally likely in both directions, the majority "bleeds" faster. No matter where you start, you converge to 50-50.
When one trait is stickier, change flows one way. Fast at the start, slowing as the favoured trait dominates. Classic r-shaped diffusion.
Unlike drift, mutation rates don't depend on N. A trait spreads at the same pace whether the population is 100 or 10,000.