Researchers at the University of East Anglia (UEA) found the length of time between product or service consumption and posting affects the review given.

For example, the sooner customers post a review the more they 'zoom-in' on their experience, focusing on the more concrete aspects, even on small details, and the more negative they are. As the time increases, reviewers 'zoom-out' and have a more positive view, focussing instead on the general experience and more abstract aspects.

With services such as hotels -- the focus of this study -- the geographical distance between the hotel and visitor's country of residence, and the cultural distance between the countries also influence how they rate their experience.

Online sales of products and services already account for a significant percentage of total retail sales and in some sectors, such as hospitality, it is much higher. When deciding what to choose from a huge variety of almost identical products online, consumers often rely on reviews from previous customers.

Published in the International Journal of Contemporary Hospitality Management, the research analysed more than 215,000 online reviews provided on TripAdvisor and Booking.com by visitors from approximately 90 countries, for 1022 London hotels.

Lead author Panagiotis Stamolampros, a PhD researcher at UEA's Norwich Business School, said: "People have time and cognitive constraints that regulate their decisions, and compared to conventional transactions in bricks and mortar stores, their ability to directly evaluate the product quality is much more limited when buying online.

"This is where online reviews come in as a tool to potentially reduce customers' risk and uncertainty and to help them make the correct product choice. However, the question remains, should online reviews be trusted?"

There have been concerns over the trustworthiness of online reviews, with consumer protection agencies trying to tackle the problem of fake reviews, policymakers developing legal frameworks and e-commerce platforms investing money to create efficient algorithms to eliminate such issues. Despite this, the study authors say online reviews should be taken with "a pinch of salt" because of the psychological influences.

"Reviewers may not intend to mislead you, but it could happen as people's memories of events are inconsistent with their perception at the time they happen," said Mr Stamolampros. "This is explained by how the human brain works and primarily related to the recall mechanisms that consumers use when evaluating an experience."

The researchers found the greater the distance between the home country of the reviewer and the country visited, the more positive the rating.

"We believe this is to do with the feeling that the more distant you are from your point of reference, which is connected with all the petty annoyances that you have to deal with in daily life, the more positive you are," said Mr Stamolampros.

However, in the case of cultural distance, the relationship is the reverse, explained co-author Dr Nikolaos Korfiatis: "Distance does not always lend enchantment to the view. People like the idea of visiting different cultures but do not always tolerate surprises.

"We are raised and formed under the social behaviour and norms that characterise our countries. When we meet something unexpected, that we are not used to, and perhaps not acceptable in our own culture, then the natural thing is to be negative towards it."

Mr Stamolampros added: "People will continue to buy products and services online as faster, cheaper delivery processes are developed, and easy return and refund processes reduce the risk. As such, people will continue to consult online reviews when making their decisions.

"However, we should understand that the inherent factors we examine here affect how individuals form their expectations and evaluations. Not all opinions expressed online will match customers' personal preferences and we may come across things that are not compatible with the expectations formed from reading the reviews. Consumers should therefore take what reviews say with a pinch of salt."

The authors say the findings have implications for both consumers and managers. They recommend that consumers should look for reviews from people who are culturally close to them, as they will be more representative of what to expect.

Managers wanting to improve service should aim to get reviewer feedback as soon as possible. Hotel managers could also be more proactive in communicating possible cultural differences or acceptable behaviours. For example, through information provided online beforehand or at check-in, as well as training for employees on cross-cultural understanding.