While personal data is a source of competitive advantage, businesses should consider the potential reaction of individuals to certain types of data requests. Privacy research has identified some factors that impact privacy perceptions, but these have not yet been linked to actual disclosure behaviour. We describe a field-experiment investigating the effect of different factors on online disclosure behaviour. 2720 US participants were invited to participate in an Amazon Mechanical Turk survey advertised as a marketing study for a credit card company. Participants were asked to disclose several items of personal data. In a follow-up UCL branded survey, a subset (N=1851) of the same participants rated how they perceived the effort, fairness, relevance, and sensitivity of the first phase personal data requests and how truthful their answers had been. Findings show that fairness has a consistent and significant effect on the disclosure and truthfulness of data items such as weekly spending or occupation. Partial support was found for the effect of effort and sensitivity. Privacy researchers are advised to take into account the under-investigated fairness construct in their research. Businesses should focus on non-sensitive data items which are perceived as fair in the context they are collected; otherwise they risk obtaining low-quality or incomplete data from their customers.