[antillean]

It's worse than that. As the next paragraph says:

"The online data were collected in two stages. In the first stage, the participants were recruited via several open internet discussion forums and several student mailing lists. In this stage, data about religious and paranormal beliefs, systemizing, mechanical abilities, and core ontological confusions were collected. The rest of the data were collected 1.5 years later. The recruitment message was sent by email to all individuals who had participated in the first stage and who had given their email address for participating in further studies (N= 1537). Of them, 237 could not be contacted because of outdated email address, and 887 did not take part in the present study."

So this is a self-selecting study: the initial group of 1537 (which was self-selected) was whittled down (largely through self-selection) to the final 258 on whom the results are based. Further to that, the authors gave no information on which of the "several open internet discussion forums and several student mailing lists" they recruited or how they did that recruitment. And they made no comparison between the demographic makeup of their sample and the general Finnish population. (Probably because it would've counted against the validity of the study since, y'know, Finland isn't 63% female or 44% students.)