Bandwagon Effect Definition:
The Bandwagon Effect is a psychological phenomenon where the likelihood of adopting an idea, belief, product, trend or behaviour is directly related to the uptake (or perceived uptake) among others. As more people take up something others jump on the bandwagon.
The bandwagon effect is a types of copying that involves social influence. Our herd instinct is a powerful driver of behaviour. Mark Earls and his co-authors in ‘I’ll have what she’s having‘ identified four decision styles that dominate most markets, two of which are heavily influenced by social influence.
Directed copying is prominent where there are relatively few similar options and people rely on authoritative individuals to follow. Undirected copying occurs in markets where there are too many choices and people to copy directly. People tend to follow the most popular choice and often with little conscious thought. This is most similar to the bandwagon effect as people copy without much, if any, conscious thought.

Resources:
Conversion marketing – Glossary of Conversion Marketing.
Herd instinct – Are most purchase decisions the result of social influence?
Herd instinct – How do social networks influence human behaviour?
Herd instinct – What makes social networks tick?
Word of Mouth – 6 myths about word of mouth marketing.
Over 300 tools reviewed – Digital Marketing Toolbox.
A/B testing software – Which A/B testing tools should you choose?
Types of A/B tests – How to optimise your website’s performance using A/B testing.