Web panels are widely employed to conduct marketing research surveys, yet little is known regarding why consumers join web panels or participate in web surveys. The present research investigated the effects of individuals' motivational traits on whether they joined web panels, participated in surveys upon joining, and the effort they put into their responses. A longitudinal study employing population profiling gathered personality measures from the entire population of potential panelists (N = 751) and invited them to join a web panel. Those accepting (N = 503) were sent a series of six marketing research surveys. Results revealed that consumers' need for cognition, curiosity, agreeableness and extraversion were significant predictors of joining the web panel. The first three traits also predicted survey participation, as did openness to experience. Among participants, response effort was affected the greatest by curiosity, extraversion, and conscientiousness. An additional experiment, conducted with 327 participants, ruled out a selection bias explanation for some results. These findings provide useful insights to researchers using web panels, and point out limitations with using strictly demographics-based weighting schemes when selecting web panels.