This paper investigates trading behaviour among Thai retail investors in 2016. Using detailed survey data from 491 investors, we examine the characteristics and behavioural patterns that lead to investor bias. Empirical results in the behavioural finance literature indicate that retail investors may not behave reasonably. Behavioural biases may influence investor decisions and affect financial markets. These studies, however, are limited to subsamples of the overall investor groups studied and mainly focus on developed markets. We find that biases are common among investors and that men are more overconfident than women. Moreover, we discover that investors with more experience in trading are less likely to hold their stocks for long periods of time. Further, investors aged 45 and younger hold more diversified portfolios. Another finding is that participants with an income of more than 50,000 Baht a month and/or who employ a number of brokers hold more diversified portfolios. This evidence is consistent with the findings that have been reported for Turkey, India, and Vietnam, indicating that demographic factors are useful for distinguishing between investors in terms of the level of overconfidence bias they exhibit. This result confirms that demographic factors play a role in differentiating and classifying retail investors and should motivate future researchers to consider these factors in their research.