Most psychology research uses opportunity samples composed of university undergrads. Undergrads are cheap, meaning you can usually force them to participate in studies for course credit, and usually in want of money - you can pay them a few pounds an hour for their time. Recently a large meta-analysis has illustrated the danger of extrapolating findings from studies using undergrad students only to the general population - most of these studies are lacking in external validity.
The most prestigious journals use students from the WEIRD - Western Educated Industrialized Rich Democratic - countries which make up less than 12% of the global population. Most of the samples used are not representative of the world we live in so there are cultural differences that are often lost that could greatly affect the results. For example, a phenomenon known as the Muller-Lyer illusion (see below) usually concludes that when lines of equal length yet the arrows point in different directions, one is perceived as shorter than the other. However, the San foragers of the Kalahari do not perceive a difference in length between the two lines suggesting the difference in environment and its surroundings plays a role in our perception of objects.