The research experiment conducted by Shawn Carlson, “A double-blind test of astrology,” published in the science journal Nature in 1985 as an indictment of astrology, is one of the most frequently cited scientific studies to have claimed to refute astrology. A Google search for the title as a quoted string returns over 6,600 links.1 Although the Carlson study drew initial criticism for numerous flaws when it was published, a more recent examination has found evidence that the study actually supports the claims of the participating astrologers. This support lends further credence to the effectiveness of ranking and rating methods, which have been used in other, lesser known astrological experiments.
The Carlson astrology experiment was conducted between 1981 and 1983 when Carlson was an undergraduate physics student at the University of California at Berkley under the mentorship of Professor Richard Muller. The flaws that have been uncovered in the Nature article include not only the omission of literature on similar studies, which is expected in all academic papers, but more serious irregularities such as skewed test design, disregard for its own criteria of evaluation, irrelevant groupings of data, removal of unexpected results, and an illogical conclusion based on the null hypothesis.
In concept and design, the Carlson experiment was not original. It was modeled after the landmark double-blind matching test of astrology by Vernon Clark (Clark, 1961). In that test astrologers were asked to distinguish between each of ten pairs of natal charts. One chart of each pair belonged to a subject with cerebral palsy and the other belonged to a subject with high intelligence. Another influential study was the “Profile Self-selection” double-blind experiment, which was led by the late astrologer Neil Marbell and privately distributed among contributors in 1981 before its eventual publication (Marbell, 1986-87). In that test, participating volunteers were asked to select their own personality interpretations, both long and short versions in separate tests, out of three that were presented.
In both of these prior studies, the participants performed well above significance in support of the astrological hypothesis as compared to chance. The Marbell study was extraordinarily qualified as it involved extensive input and review from astrologers, scientists, statisticians, and prominent skeptics. Carlson neglected to provide any review of these scientific studies that supported astrology or any other previous related experiments.
The stated purpose of Carlson’s research was to scientifically determine whether the participating astrologers (members of the astrology research organization NCGR and others) could match natal charts to California Psychological Inventory (CPI) profiles (18 personality scales generated from 480 questionnaire items). Additionally, Carlson would determine whether participating volunteers (undergraduate and graduate students, and others) could match astrological interpretations, written by the participating astrologers, to themselves. These assessments, Carlson asserts, would test the “fundamental thesis of astrology” (Carlson, 1985: 419).
From the time of its release, the Carlson study has been criticized for the extraordinary demands it placed on the participating astrologers, which would be regarded as unfair in normal social science. As with any controversial study, all references to Carlson’s experiment should include the scientific discourse that followed it, particularly the points of criticism that show weaknesses in the design and analysis. Notable among recent critics has been University of Göttingen emeritus professor of psychology Suitbert Ertel, who is an expert in statistical methods and is known for his criticism of research on both sides of the astrological divide. Ertel published a detailed review in a 2009 article, “Appraisal of Shawn Carlson’s Renowned Astrology Tests” (Ertel, 2009).
From a careful reading of Carlson’s article in light of the ensuing body of discourse, we can appreciate that the design of the experiment was intentionally skewed in favor of the null hypothesis (no astrological effect), which Carlson refers to, somewhat misleadingly as the “scientific hypothesis.” Some of the controversial features of the design are as follows:
• The astrologers were not supplied with the gender identities of the CPI owners, even though the CPI creates different profiles for men and women. (Eysenck, 1986: 8; Hamilton, 1986: 10).
• Participants were not provided with sufficiently dissimilar choices of interpretations, as the Vernon Clark study had done, but instead were given randomly selected choices. This may give the impression of a fair method, but given the narrow demographics of the sample, there is an elevated likelihood of receiving similar items from which to choose, which makes it unfair (Hamilton, 1986: 12; Ertel, 2009: 128).
• The easier to discriminate and more powerful two-choice format, which had been used in the Vernon Clark study, was replaced with a less powerful three-choice format, which further elevated the chances of receiving similar items (Ertel, 2009: 128). No reasons are given for this unconventional format, although it can be surmised that Carlson was well aware of the complexities of a three-choice format from his familiarity with the Three-Card Monte (“Follow the Lady”) sleight of hand confidence game, which he had often played as a street psychic and magician (Vidmar, 2008).
• The requirement for rejecting the “scientific hypothesis” was elevated to 2.5 standard deviations above chance. In the social sciences, the conventional threshold of significance is one standard deviation (Z = 1.64, with probability less than p = .05) (Ertel, 2009: 135).
• Failure to consider the astrologers’ methodological suggestions or give an account of their objections. Carlson credits astrologer Teresa Hamilton with giving “valuable suggestions,” yet Hamilton complained later that “Carlson followed none of my suggestions. I was never satisfied that the experiment was a fair test of astrology” (Hamilton,1986: 9).
Given this skewed design, the irregularities of which are not obvious to the casual reader, Carlson directs our attention to the various safeguards he used to assure us that no unintended bias would influence the experiment. He describes in detail the precautions used to screen volunteers against negative views of astrology, how the samples were carefully numbered and guarded to ensure they were blind, and the exact contents of the sealed envelopes provided to test participants.
The experiment consisted of several separate tests. The astrologers performed two tests, a CPI ranking test and a CPI rating test. The volunteer students performed three tests, a natal chart interpretation ranking test, a natal chart interpretation component rating test, and a CPI ranking test.
In the CPI ranking test, astrologers were given, for each single natal chart, three CPI profiles, one of which was genuine, and asked to make first and second choices. There were 28 participating astrologers who matched 116 natal charts with CPIs. Success, Carlson states, would be evaluated by the frequency of combined first and second choices, which is the correct protocol for this unconventional format. He states, “Before the data had been analyzed, we had decided to test to see if the astrologers could select the correct CPI profile as either their first or second choice at a higher than expected rate” (Carlson, 1984: 425).
In addition to the ranking test of first, second, and third best fit, the astrologers were tested for their ability to rate the same CPIs according to a scale of accuracy. This task allowed for finer discrimination within a greater range of choices. Each astrologer “also rated each CPI on a 1-10 scale (10 being the highest) as to how closely its description of the subject’s personality matched the personality description derived from the natal chart” (Carlson, 1985: 420).
As to the results of the astrologers’ three-choice ranking test, Carlson first directs our attention to the frequency of the individual first, second, and third CPI choices made by the astrologers, each of which he found to be consistent with chance within a specified confidence interval. This observation is scarcely relevant, given the stated success criteria of the first and second choice frequencies combined. Then, to determine whether the astrologers were successful, Carlson directs our attention to the rate for the third place choices, which, as already noted, was consistent with chance. Thus he declares that the combined first two choices were not chosen at a significant frequency.
“Since the rate at which the astrologers chose the correct CPI as their third place choice was consistent with chance, we conclude that the astrologers were unable to chose [sic] the correct CPI as their first or second choices at a significant level” (Carlson, 1984: 425). This conclusion, however, ignores the stated success criteria and is in fact untrue. The calculation for significance shows that the combined first two choices were chosen at a success rate that is marginally significant (p = .054) (Ertel, 2009: 129).
As to the results of the astrologers’ rating test (10-point rating of three CPIs against each chart), Carlson demonstrates that the astrologers’ ratings were no better than chance within the first, second, and third place choices made in the three-choice test. He shows a weighted histogram and a best linear fit graph to illustrate each of these three groups of ratings. Carlson directs our attention to the first choice graph as support for his conclusion for this test. The slope of this graph is “consistent with the scientific prediction of zero slope” (Carlson, 1985: 424). The slope is actually slightly downward. The graphs for the other two choices are not remarked upon, but show slightly positive slopes.
The problem with Carlson’s analysis of the 10-point rating test, however, is that this test had no dependency on the three-choice ranking test and even used a different sample size of CPIs.2 According to the written instructions supplied to the astrologers, this rating test was actually to be performed before the three-choice ranking test (Ertel, 2009: 135). These 10-point ratings should not be grouped as though they were quantitatively related to the later three-choice test. Confirmation bias from the claimed “result” of the three-choice test, which Carlson presents earlier in his paper, suggests acceptance of irrelevant groupings in this 10-point rating test, presented later. When the totals of the ratings are considered without reference to the choices made in the subsequent test, a positive slope is seen, which shows that the astrologers actually performed at an even higher level of significance (p = .037) than the three-choice test (Ertel, 2009: 131).
The other part of Carlson’s experiment tested 83 student volunteers to see if they could correctly choose their own natal chart interpretations written by the astrologers. Volunteers were divided into a test group and a control group. Members of the test group were each given three choices, all of the same Sun sign, one of which was interpreted from their natal chart (Carlson, 1985: 421). Similarly, each member of the control group received three choices, all of the same Sun Sign, except none of the choices was interpreted from their natal charts, although one choice was randomly selected as “correct” for the purpose of the test.
For the results of this test, Carlson shows a comparison of the frequencies of the correct chart as first, second, and third choices for the test group and the control group (again ignoring his stated protocol to combine the frequencies of the first two choices). He finds that the results are “all consistent with the scientific hypothesis” (Carlson, 1985: 424). However, he does note an unexpected result for the control group, which was able to choose the correct chart at a very high frequency. He calculates this to be at 2.34 standard deviations above chance (p = .01). Yet, because this result occurred in the control group, which was not given their own interpretations, Carlson interprets this as a “statistical fluctuation.”
Yet the size of this statistical fluctuation is so unusual as to attract skepticism, particularly in light of Carlson’s other results. It is reasonable to think that the astrologers could write good quality chart interpretations after having successfully matched charts with CPI profiles. Yet, according to Carlson’s classification, the test group tended to avoid the astrologers’ correct interpretations and choose the two random interpretations, while the control group tended to choose the selected “correct” interpretations by a wide margin, as if they, the controls, had been the actual test subjects (Ertel, 2009: 132). This raises suspicion that the data might have been switched, perhaps inadvertently, but this is unverifiable speculation (Vidmar, 2008).
Like the participating astrologers, the student volunteers were also given a rating test; in this case for the sample chart interpretations they were given. They were asked to rate, on a scale of 1 to 10, the accuracy of each subsection of the natal chart interpretations written by the astrologers. “The specific categories which astrologers were required to address were: (1) personality/temperment [sic]; (2) relationships; (3) education; (4) career/goals; and (5) current situation” (Carlson, 1985: 422). This test would potentially have high interest to astrologers because of the distinction it made between personality and current situation, which is a distinction that is not typically covered in personality tests. Also, the higher sensitivity of a rating test could provide insight, at least as confirmation or denial, into the extraordinary statistical fluctuation seen in the three-choice ranking test.
However, based on a few unexpected results, Carlson decided that there was no guarantee that the participants had followed his instructions for this test. “When the first few data envelopes were opened, we noticed that on any interpretation selected as a subject’s first choice, nearly all the subsections were also rated as first choice” (Carlson, 1985: 424). On the basis of this unanticipated consistency, Carlson rejected the volunteers’ rating test without reporting the results.
As an additional test in this part of the experiment, the student volunteers were asked to choose from among three CPI profiles the one that was based on the results of their completed CPI questionnaire. The other two profiles offered were taken from other student volunteers and randomly added. Of the 83 volunteers who completed the natal chart interpretation choices, only 56 completed this task. As usual, Carlson compared the results of the three choices for the test and control groups taken individually (instead of the frequency of the first two choices taken together). Furthermore, in contravention to the logic of control group design, Carlson compares the two groups against chance instead of against each other (Ertel, 2009: 132). He found no significant difference from chance for the two groups.
There are plausible reasons that could explain why the test group was unable to correctly select their own CPI profiles, even though the astrologers were able to a significant extent as we have seen, to match CPI profiles with the students’ charts. The disappointing number of students who completed this task, despite having endured the 480-question CPI questionnaire, suggests that the students might have been much less motivated than the astrologers, for whom the stakes were higher (Ertel, 2009: 133).
The CPI matching tasks, for both the volunteers and the astrologers, were especially challenging because of the three-choice format. The random selections of CPIs made within the narrow demographics of the sample population of students would have elevated the likelihood of receiving at least two CPI profiles that were too similar to make a discriminating choice and this would have had a negative impact on motivation.
Despite its numerous flaws and unfair challenges, the Carlson experiment nevertheless demonstrates that the astrologers, in their two tests, were able to match natal charts with CPI profiles significantly better than chance according to the criteria normally accepted by the social sciences. Thus the null hypothesis must be rejected.3 As such, the Carlson experiment demonstrates the power of ranking and rating methods to detect astrological effects, and indeed helps to raise the bar for effect size in astrological studies. The benchmark effect size that had been attained by the late astrological researcher Michel Gauquelin was merely .03 to .07. Although these were small effects, they were statistically very significant due to large sample sizes (N = 500-1000 or more natal data) and had to be taken seriously (Gauquelin, 1988a). In Carlson’s experiment, which applied sensitive ranking controls, the effect size of the three-choice matching test with p = .054 is ES = .15, and the effect size of the 10-point rating test with p = .037 is ES = .10 (Ertel, 2009: 134).
The evidence provided by the Carlson experiment, when considered together with the scientific discourse that followed its publication, is extraordinary. Given the unfairly skewed experimental design, it is extraordinary that the participating astrologers managed to provide significant results. Given the irregularities of method and analysis, which had somehow remained transparent for 25 years, it is extraordinary that investigators have managed to scientifically assess the evidence and bring it into the full light of day. Now that the irregularities have been pointed out, it is easy to see and appreciate what Carlson actually found.
However, because of the unfairness and flaws in the Carlson experiment, this line of research needs to be replicated and then potentially extended in further studies. If natal charts can be successfully compared with self-assessment tests, as the Carlson experiment indicates, then astrological features might be easier to evaluate than was previously believed. New questions must now be raised. What would the results be in a fair test? Why did the astrologers choose and rate the CPIs as they did? Which chart features should be compared against which CPI features? Could more focused personality tests provide sharper insights and analysis? The door between astrology and psychology has been opened by a just crack and we have caught a glimpse of hitherto unknown connections between the two disciplines.
1. By comparison, a Google perusal of some other peer reviewed journal articles on astrology, searched as quoted strings, returns the following: “Is Astrology Relevant to Consciousness and Psi?” (Dean and Kelly, 2003) 8800 hits; “Are Investors Moonstruck?—Lunar Phases and Stock Returns” (Yuan et al, 2006) 3700 hits; “Objections to Astrology: A Statement by 186 Leading Scientists” (The Humanist, 1975) 3500 hits; “A Scientific Inquiry Into the Validity of Astrology” (McGrew and McFall, 1990) 2160 hits; “Raising the Hurdle for the Athletes’ Mars Effect” (Ertel, 1988) 1350 hits; “The Astrotest” (Nanninga, 1996) 970 hits; “Is There Really a Mars Effect?” (Gauquelin, 1988) 630 hits.
2. Carlson presents the 10-point rating test as a finer discrimination of the 3-choice ranking test, but the sample size is not the same. A sample of 116 natal charts is used in the 3-choice test (Carlson, 1985: 421, 423) and a different sample size is used for the 10-point rating test, which adds to the discrepancies already mentioned between these two tests and further emphasizes that they cannot be considered as a single test. Carlson does not give the sample size for the 10-point test, but it can be determined by measurement of the first, second, and third choice histograms in his article (Carlson, 1985: 421, 424). Each natal chart had to be the “correct” choice in one of these three “choices.” By adding up these “correct hits,” Ertel shows 99 charts (Ertel, 2009: 130, Table 3). A more exacting scrutiny of the histograms by Robert Currey (in a forthcoming article) determines 100 charts.
3. Carlson had concluded: “We are now in a position to argue a surprisingly strong case against astrology as practiced by reputable astrologers” (Carlson, 1985: 425). Ertel points out the logical flaw that such a conclusion cannot be drawn even if the tests had shown an insignificant result. “Not being able to reject a null hypothesis does not justify the claim that the alternate hypothesis is wrong” (Ertel, 2009: 134).
Carlson, Shawn (1985). “A double-blind test of astrology,” Nature, (318): 419-425. http://muller.lbl.gov/papers/Astrology-Carlson.pdf, retrieved on 2010-12-04.
Clark, Vernon (1961). “Experimental astrology,” In Search, (Winter/Spring): 102-1 12.
Ertel, Suitbert (1988). “Raising the Hurdle for the Athletes’ Mars Effect: Association Co-varies with Eminence” Journal of Scientific Exploration, (2): 4.
Ertel, Suitbert (2009). “Appraisal of Shawn Carlson’s Renowned Astrology Tests,” Journal of Scientific Exploration, (23): 2.
Eysenck, H.J. (1986). “A critique of ‘A double-blind test of astrology’” Astropsychological Problems, 1(1): 27-29.
Hamilton, Teressa (1986). “Critique of the Carlson study” Astropsychological Problems, (3): 9-12.
Marbell, Neil (1986-87). “Profile Self-selection: A Test of Astrological Effect on Human Personality,” NCGR Journal, (Winter): 29-44.
McGrew, John H. and Richard M. McFall (1990). “A Scientific Inquiry Into the Validity of Astrology,” Journal of Scientific Exploration, (4) 1: 75-83.
Vidmar, Joseph (2008). “A Comprehensive Review of the Carlson Astrology Experiments,” http://www.astronlp.com/Carlson Astrology Experiments.html, retrieved on 2010-08-01.