Facebook has considered profiling its users' personalities and using the information to target adverts.

A patent filed by the social network describes how personality characteristics,including emotional stability, could be determined from people's messages and status updates.

The firm is currently embroiled in a privacy scandal over the use of its data by a political consultancy.

Facebook says it has never used the personality test in its products.

The patent, first filed in 2012, is in the names of Michael Nowak and Dean Eckles.

Mr Nowak has worked for Facebook for 10 years, while Prof Eckles now teaches at the Massachusetts Institute of Technology.

The patent has been updated twice, most recently in 2016.

'Status updates'

The BBC has seen emails from Mr Eckles and other Facebook staff to University of Cambridge psychologists in which they discuss analysis of data to infer personality traits, and talk of using such research to improve the product for users and advertisers.

The political consultancy Cambridge Analytica claimed that it used a similar technique, known as psychographics, in its work, though it has denied using data sourced from a Facebook personality quiz in the 2016 US presidential election.

Facebook's patent says potential sources of data could include "status updates, notes, messages, posts, comments, or any other communications from which linguistic data may be extracted".

It says the personality characteristics could then be stored in a user's profile and used "to select news stories, advertisements, or recommendations of actions presented to the user".

Prof Eckles told the BBC that his research had involved asking Facebook users to complete surveys that posed personality questions, adding that it had been made clear that the social network was behind the questionnaires.

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He acknowledged that "psychographic [advertising] targeting raises some distinctive ethical concerns" but said he was doubtful it would ever have been effective.