Ogilvy Action surveyed more than 14,000 shoppers in 24 countries around the world about how they buy a number of different product categories, including shampoo, coffee and cigarettes as well as beer.
It found that nearly 10% of people in the UK who buy beer don't go for the brand they planned to buy once they are in shops.
"Beer was the most dynamic category," said Jordi Connor, head of information & insight at Ogilvy Action. "When you speak to people pre-shop there is often a brand that gets a disproportionate amount of people saying they are going to buy it - but that is invariably not the winner in-store.
"That shows that brands are doing things in-store and it can make a huge difference to how they do."
10% of shoppers didn't buy the brand they planned to buy
5% of shoppers who didn't plan to buy beer did buy it
36% of people bought more than they planned to
Men aged 35-44 were less brand loyal than other ages - across all categories, not just beer
Supermarket shoppers are much more likely to stick to their planned brand; shoppers in independents are much more likely to decide what to buy in-store.