In decision research, it has been accepted that investigating the cognitive processes underlying human behaviours is as important as studying choices and judgements themselves (Johnson et al., 2008; Schulte-Mecklenbeck et al., 2017). In this thesis, I present my findings on the psychological processes of well-known phenomena in two subfields of behavioural science, including multialternative choices and public opinions. For multialternative choices, with two process-tracing methods, I examined how processes at different cognitive levels contribute to producing the attraction effect (Huber et al., 1982; Simonson, 1989), which resulted in the first two papers in my thesis. For public opinions, by analysing the behaviours of individuals on online platforms over time, I investigated how psychological processes related to echo chambers (Sunstein, 2001, 2007) and confrontation (Karlsen et al., 2017) influenced online engagement and discussed their implications for political polarisation. The third paper in my thesis was developed based on these results.