In June we tested 52 participants, which was our minimum. The overall results showed evidence for the null hypothesis, albeit it was not very compelling. We believe that if we test at least 48 more participants (bringing it to a total of 100) our results will be clear enough to draw strong conclusions regarding the competing hypotheses. Therefore we will continue our research this semester. As stated above our goal is to test at least 100 participants (as Bem tested 100 participants in his experiment).We will monitor the Bayes factor as the participants come in and stop whenever the result is clear enough.
The experiment will of course be exactly the same as last semester. Participants will first fill out a questionnaire, then watch a relaxing video and finally perform the actual task on a computer. Due to a randomization issue, our first 52 participants were not yoked, but each received a completely randomized sequence. For purposes of uniformity, we will continue the fully randomized sequence for the new participants.
We also want to attach a paper we have written over the interim results. It is in Dutch.
Interim results (in Dutch)
Hi,
BeantwoordenVerwijderenFirstly, I applaud your attempt to replicate Bem's experiments. However, I was reading your methods pdf and was slightly alarmed that you decided to only test women. You say that this is because Bem claimed that the erotic IAPS stimuli are only reactive for women. Where do you think Bem makes this claim in his paper? As far as I can see, his paper only says that men do not react to the negative stimuli.
For example, in Experiment 5 (of Bem's paper) it says this:
"women achieved a significant hit rate on the negative pictures...but men did not".
In Experiment 6 it says this:
"As noted above, women showed a significant psi effect on the negative trials in Experiment 5, but men did not. Because the psi literature does not reveal any systematic sex differences in psi ability, it seemed possible that the men were simply less aroused than the women by the negative pictures. The ratings supplied with the IAPS pictures revealed that male raters rated every one of the negative pictures in the set as less negative and less arousing than did female raters. Also, an fMRI study using IAPS pictures found that men had significantly fewer brain regions than women where activation correlated with concurrent ratings of their emotional experience. So, for this replication, we supplemented the IAPS pictures for men with stronger and more explicit negative and erotic images obtained from Internet sites."
In Experiment 1, Bem summarises the changes he made as a result of Experiment 5 saying,
"in our first retroactive experiment (Experiment 5, described below), women showed psi effects to highly arousing stimuli but men did not. Because this appeared to have arisen from men’s lower arousal to such stimuli, we introduced different erotic and negative pictures for men and women in subsequent studies, including this one, using stronger and more explicit images from Internet sites for the men."
Therefore, it appears that although Bem did supplement male stimuli with stronger and more explicit erotic images, that decision does not seem to have been based on a sex difference in reactivity to erotic stimuli.
In fact, it is unclear to what extent women were reactive to the erotic material contained in the IAPS set compared to men (Bem is clear on the differences in reactivity to negative stimuli). Perhaps you should consider including men in future studies, supplementing stimuli with more erotic and explicity pictures as Bem did? I would guess that men are generally more reactive to such material.
Thanks