How and why do people share opinions online? In research conducted offline, the social identity of the audience is a key factor: whether they are composed of one’s ingroup or outgroup affects what people share and why. Do people behave similarly and for similar reasons online? To test this, we put participants (N = 326) in imaginary forums belonging to their ingroup and outgroup. In each, people selected statements to share, along with reasons for doing so. The results showed a high degree of heterogeneity; people shared nearly all kinds of statements with both audiences, for a variety of reasons. However, there were also consistent patterns. Identity expression was the most common reason for sharing to both audiences, but this led to different things being shared to each. To the ingroup, people preferred to share statements expressing ingroup beliefs, while to the outgroup, they preferred statements expressing universal beliefs.

People have different intentions when sharing information online. However, are others able to interpret these motives and form accurate impressions of the poster? To investigate this, we put participants (N = 307) in imaginary opinion-based ingroup and outgroup online forums. In each, people were presented with different types of statements and asked for their impressions of the poster as well as of their own ingroup and outgroup. Negative impressions and intentions were more commonly linked to posters thought to be outgroup members, even when they exhibited similar behaviours to posters thought to be members of the ingroup. Notably, most types of contact increased people's liking of the ingroup and disliking of the outgroup. That said, a perceived effort to engage in genuine discussion over group matters by perceived outgroup posters appeared to shift outgroup impressions to be more positive. This highlights the potential benefit of deliberation in mitigating intergroup animosity.