Response to Alcock’s “Back from the Future: Comments on Bem”

Daryl J. Bem

Daryl J. Bem, professor emeritus of psychology at Cornell University, responds to a critique of his recent article on his parapsychological research.

Note: This post is a response to James Alcock’s article, which may be viewed here.

On December
3, 2010, James Alcock published an essay on this site critiquing my
article “Feeling the Future: Experimental Evidence for Anomalous Retroactive
Influences on Cognition and Affect,” which has been accepted for publication
in the Journal of Personality and Social Psychology (JPSP).
(A prepublication copy is available at Even though the article will not
appear in print for several months, it began to receive widespread media
coverage after the experiments were described by a blogger for Psychology

Alcock begins
his critique by noting that

“What has made
this report so particularly newsworthy is both the academic stature
of the author, a respected Professor of Psychology at Cornell University,
and the fact that it is published in the American Psychological Association’s
(APA) Journal of Personality and Social Psychology,
the world's preeminent social psychology journal.”

I believe
that Alcock has also put his finger on what is so particularly newsworthy
about his critique: the striking contrast between his harsh assessment
of my work and the collective assessment of the two editors and four
reviewers who vetted it for the Journal. JPSP in one of the most
rigorously refereed journals in the entire field of psychology, with
a rejection rate of 82% in 2009. Moreover, authors’ names and other
identifying information are removed from a manuscript before it is sent
to reviewers so that their evaluations will be based purely on its merits
and not be influenced by knowledge of an author’s reputation or the
prestige of his or her institutional affiliation.

The contrast
between the assessments of Alcock and the Journal’s editors and reviewers
is also particularly newsworthy because it is not simply a reprise of
the familiar disagreement between skeptics and proponents of psi (ESP).
Like Alcock, several of the reviewers expressed various degrees of skepticism
about the reality of psi, while still urging its acceptance. Unlike
Alcock, however, they are all active researchers who regularly contribute
to the mainstream experimental literature in psychology and cognitive
science. Their task was to evaluate the logic and clarity of the article’s
exposition, the soundness of its experimental methods, and the validity
of its statistical analyses. They did not have to agree with my conclusions
regarding psi to make those assessments. As Joachim Krueger, an experimental
psychologist at Brown University, put it so charmingly: “My personal
view is that this is ridiculous and can’t be true. Going after the
methodology and the experimental design is the first line of attack.
But frankly, I didn't see anything. Everything seemed to be in good
order (quoted by Peter Aldhous in the NewScientist 16:29, November
11, 2010).”

The Research

My article
reports nine experiments, involving more than 1,000 participants, that
test for precognition or retroactive influence by “time-reversing”
well-established psychological effects so that the individual’s responses
are obtained before rather than after the stimulus events occur. Each
time-reversed experiment tests the straightforward hypothesis that we
should observe the same effect that we normally observe in the standard
(non-psi) version of the experiment. Five different effects are tested
in this way; and, to bolster confidence in the results, four of the
nine experiments are actually replications of the other experiments
in the article. Across all nine experiments, the combined odds against
the findings being due to chance are greater than 70 billion to 1.

The Critique

Alcock challenges
both my experimental procedures and my statistical analyses. His article
is quite lengthy, and so I will here focus only on his two most frequently
recurring criticisms, one concerning experimental procedures and one
concerning statistical analyses. (I will not here address Alcock’s
lengthy preamble in which he imaginatively rewrites the history of psi
research. That has already been done by Dean Radin on
his Internet blog at

major procedural criticism concerns my selection and deployment of the
pictorial stimuli used in six of the nine experiments. As explained
in the article, they are drawn primarily from the widely used International
Affective Picture System (IAPS), a set of 820 digitized photographs
that have been rated by both male and female raters on numerical scales
for their emotional tone (extremely negative to extremely positive)
and their arousal level (non-arousing to highly arousing).

Male and
female raters differed markedly in their ratings of negative and erotic
pictures. Male raters rated every one of the negative pictures as less
negative and less arousing than did the female raters, and they gave
more positive ratings than the female raters to the most explicit erotic
pictures. Possibly reflecting this sex difference, female participants
showed significant psi effects with negative and erotic stimuli in my
earliest experiment but male participants did not. Accordingly, I decided
to introduce different sets of pictures for men and women in subsequent
experiments, choosing more extreme and more arousing pictures for the
men. As a result, no sex differences in psi performance appeared in
any of the later experiments. In addition, the computer program gave
participants the choice of being shown either opposite-sex or same-sex
erotic pictures, without having their choice divulged to the experimenter.

the JPSP reviewers had no problems with any of this, Alcock clearly
does: “Now we find that participants were allowed to choose their
target set! This is the most baffling description of research materials
and procedures that I have ever encountered.” I am surprised by Alcock’s
reaction here. Because he had post-doctoral training in clinical psychology
and has served as a member of the Council
for Scientific Clinical Psychology and Psychiatry, I would have expected
him to be familiar with several well-known clinically-oriented experiments
on reactions to threat in which different sets of threatening stimuli
were assembled for groups of participants with different psychiatric
diagnoses (e.g., homosexually-toned materials for male patients diagnosed
with paranoia). Some of those experiments even constructed tailor-made
sets of stimuli for each individual participant. This is all kosher.
The conceptual hypotheses in those experiments concerned the ways in
which participants responded to stimuli threatening to them.
Similarly, the hypotheses is my experiments concern the ways in which
participants respond to stimuli that are erotically arousing for

If Alcock
believes that having different sets of erotic stimuli for men and women
or for gay and heterosexual participants is a flawed procedure, then
he should spell out how and why he thinks this could possibly lead to
false positive results.
This example also illustrates
a more general problem with Alcock’s critique: A failure to distinguish
between potential flaws in an experiment that would illegitimately produce
false positive results and potential flaws that would actually work
against the experimental hypothesis by introducing noise into the data.
The first kind of flaw is fatal and constitutes grounds for rejecting
the probative value of an experiment. The second kind of flaw simply
produces weak or nonsignificant results and is a setback only for the
experimenter. For example, if using different sets of stimuli
for men and women were a flawed procedure, it would have been a flaw
of this second kind and would have militated against positive results.

The Precognitive Detection
of Erotic Stimuli

made his comment about being baffled by the description of research
materials in his discussion of the first experiment reported in the
article, and I presume he meant it to apply to other features of that
experiment as well. The experiment was designed to test the hypothesis
that individuals can precognitively detect the future location of an
erotic picture.

There were
100 sessions in this experiment, and on each of 36 trials, participants
saw images of two curtains side-by-side on the computer screen. They
were told that a picture would be behind one of the curtains and a blank
wall would be behind the other. Their task on each trial was to click
on the curtain they felt concealed the picture. After they made their
selection, the selected curtain opened, revealing either a picture or
a blank wall. Unknown to participants at the time, the computer did
not actually select the picture to be shown or determine its left/right
position until after they had already made their decision. This procedure
thus tested a participant’s ability to anticipate a future event,
a test of precognition.

On randomly
selected trials, the picture was erotic; on other trials, it was a nonerotic,
and the participant had no (non-psi) way of knowing which kind of picture
would be used on any given trial. Because there were two alternatives
on each trial—left curtain or right curtain—the probability that
the participant would correctly select the location of the picture by
chance was always 50%. (Alcock apparently misunderstood the procedure,
concluding somehow that trials with nonerotic pictures had a 33% chance
probability of success. When he says he is baffled, I believe him. Fortunately,
the reviewers for JPSP were not baffled.) Accordingly, the experimental
hypothesis being tested in this experiment was that on trials using
erotic pictures, participants would select the correct curtain on significantly
more than 50% of the trials.

hypothesis was supported: Participants successfully detected the future
location of the erotic pictures on 53.1% of the trials. This result
was evaluated for statistical significance by a t
test, which evaluated the probability that a 53.1% success rate across
100 sessions could have arisen by chance. By convention, psychologists
are permitted to call a result “statistically significant” if it
could have arisen by chance less than 5% of the time. This particular
result could have occurred by chance less than 1% of the time.

I further
analyzed the data to see if participants could also detect the future
locations of nonerotic pictures. It might well be that there is nothing
unique about erotic pictures beyond their high arousal value and positive
emotional tone. Using the IAPS numerical ratings, I defined four kinds
of nonerotic pictures: emotionally negative pictures, emotionally neutral
pictures, emotionally positive pictures, and romantic-but-nonerotic
pictures (e.g., a kiss between a bride and groom at their wedding).

To accommodate
so many different kinds of nonerotic pictures, I divided the 100 sessions
into two parts. Forty sessions included trials with negative and neutral
pictures and sixty sessions included trials with positive and romantic
pictures. By design, this yielded 600 positive trials and 480 each of
negative, neutral, and romantic trials—enough of each to permit separate
statistical testing. A t test across all sessions revealed that
that participants did no better than chance on nonerotic pictures, and
separate t tests further revealed that they did no better than
chance on any of the subsets of nonerotic pictures.

The Problem of Multiple
Statistical Tests

This brings
us to the statistical criticism that Alcock raises repeatedly throughout
his critique. As he correctly notes, it is illegitimate and misleading
to perform multiple tests on a set of data without adjusting the resulting
significance levels to take into account the number of separate analyses
conducted. This is well known to experimental psychologists, but, in
fact, it does not apply to any of the analyses in my article. Alcock
has memorized the right words about multiple tests, but does not appear
to understand the logic behind those words.

For example,
as noted above, multiple t tests demonstrated that participants
did no better than chance on any of the subcategories of nonerotic pictures.
It is here that Alcock first complains about my performing multiple
tests without adjusting the significance level for the number of tests

In this case,
Alcock is almost right. Suppose that in testing each of the four subcategories
of nonerotic pictures, I had found that one of them (e.g., romantic
pictures) showed a significant precognitive effect. Because this finding
would have emerged post hoc, only after I had first performed separate
tests on four different picture types, I would have had to adjust the
significance level to be less significant. If I did not, I would be
illegitimately capitalizing on the likelihood that at least one of the
four tests would have yielded a positive result just by chance. But
there was no psi effect on any of the subcategories of nonerotic pictures.
Perhaps Alcock wants me to change my conclusion that there were no significant
effects on nonerotic pictures to the conclusion that there were really
no significant effects on nonerotic pictures.

In choosing
to test the main hypothesis about erotic pictures in this experiment
with a t test, I was aware that particularly cautious or skeptical
readers might worry about the mathematical assumptions that underlie
this common statistical test. So, I demonstrated that the same result
would be obtained by using an alternative test—called a “nonparametric”
test—that did not rest on those assumptions. I did this throughout
the article, showing in every experiment that the same conclusions are
reached no matter which kind of test is used. These multiple tests were
thus aimed at showing that the same conclusion arises from different
statistical treatments of the same data. This is very different from
conducting several exploratory tests on different portions of the data
and then concluding post hoc that one of them showed a significant effect.
We shall now see further evidence that Alcock seems not to understand
the difference.

The Retroactive Priming

of my experiments tested retroactive priming, a time-reversed version
of a popular procedure in contemporary cognitive and social psychology.
In a typical (non-psi) priming experiment, participants are asked to
judge as quickly as they can whether a picture is pleasant or unpleasant,
and their reaction time is measured. Just before the picture appears,
a pleasant or unpleasant word (e.g., beautiful, ugly)
is flashed briefly on the screen; this word is called the prime. Individuals
typically respond more quickly when both the prime and the picture are
both pleasant or both unpleasant than when one is pleasant and the other
is unpleasant. In my time-reversed version of the procedure, the prime
did not appear until after participants made their judgments of the

objections to these experiments are that my

    …data analyses are very
    complex, involving two transformations as well as outlier cut-off criteria,
    and without access to the actual data, [it] is difficult to evaluate
    the adequacy of the analysis. However, it is obvious once
    again that multiple comparisons were carried out without any control
    for multiple testing.

With regard
to the complexity of the data analysis, it is true that reaction time
data require specialized treatment, and I adopted the analytic procedures
that are now considered standard for priming studies. The associate
editor and two of the reviewers of my article are experts in priming
studies and major contributors to the priming literature. Had I not
performed the standard analyses of the data, the reviewers would have
required me to do so before they accepted the article. At least one
expert in priming experiments has also argued that one should always
perform several analyses using different transformations and different
cut-off criteria to ensure that the priming effects hold up across these
variations. That is precisely what I did. Unlike Alcock, the reviewers
understand both the statistical treatment of priming data and why the
multiple tests strengthen the conclusions drawn.

Multiple Tests—One More

In two of
my experiments, I was concerned about potential bias or nonrandomness
in the computer's successive left/right placements of the target pictures,
so I presented four different data analyses, each one controlling in
a different ways for possible bias in the randomization process. Again,
Alcock robotically invokes his mantra about multiple tests, failing
to realize that the whole point of multiple tests in these experiments
was to demonstrate in several converging ways that my conclusions were
not compromised by bias in the random placement of target pictures.

one purpose in reporting multiple tests throughout the article was to
counter a charge often made by skeptics who are tempted to explain away
psi data on the grounds of experimenter dishonesty: This is the charge
that an experimenter might have tried out several statistical tests
and then cherry-picked among them to report only the one that worked.
Alas, when dealing with Alcock, no good deed goes unpunished.