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Understanding Astrology:
A critical review of a thousand empirical studies 1900-2020

Abstract -- Fifty years ago empirical studies (studies based on experiments) were the hardest things to find in astrology. There were only opinions. Today there are more than a thousand empirical studies hidden in a hundred journals and dozens of books, plus academic theses, conference reports, websites, unpublished studies, and little-known hard-to-find specialised collections. For the first time Understanding Astrology brings together ALL of these highly scattered studies -- not just the ones conveniently available or selected to prove a point -- and subjects them to rigorous critical thinking. It puts astrology under the microscope in a concise style free of waffle. The results explain why for many people there is more to astrology than being true or false. Understanding Astrology has 952 pages, more than 650,000 words, 750 pictures, 650 graphs, nearly 500 tables, over 4000 references, glossary (85 entries), and name and subject indexes. Two-thirds is taken up with individual descriptions of the 1000 studies. The treatment is balanced (all four authors have strong science backgrounsds, all are experienced writers, and two were once full-time professional astrologers and teachers of astrology). And there are no conflicts of interest (no author makes money from astrology or from sales of the book). The book is Open Access and can be downloaded for free, see full article below. Hardcover copies with a durable sewn binding (lies flat) are available direct from the publisher for 65 euros (abour $US60) excluding postage. The ISBN is 978908-249291-0 but to avoid high prices the book is not available from bookstores. It was published in late 2022 and also updates the present website.

The evolution of Understanding Astrology
Fifty years ago calculating a birth chart was too time-consuming for most researchers into astrological claims. As a result few empirical research studies existed. Examples included John Nelson on shortwave radio quality, Vernon Clark on the performance of astrologers, John Addey on harmonics, and Michel Gauquelin on professional occupations. All were reviewed in Recent Advances in Natal Astrology: A Critical Review 1900-1976, the result of a unique international collaboration between more than 50 astrologers and scientists from 10 countries plus the resourcs of the AA's research section. (AA = UK's Astrological Association founded in 1958 "to enlarge the knowledge of Astrology in a scientific spirit".) The 1977 publication of Recent Advances led in 1979 to the first of a series of annual London research conferences and in 1981 to the re-launching of Correlation, the AA's journal of research into astrology. The book also received international acclaim:

Cover of Recent Advances Recent Advances in Natal Astrology
It had 250,000 words, nearly 110 figures, over 1000 references. "The most important book ever written on astrology" (Phenomena Publications Canada). "A must for any serious scientific student of astrology" (Astrological Magazine India). "A milestone in astrological book publishing" (FAA Journal Australia)."The most talked-about astrology book ever" (Emergence bookshop UK). "Easily readable and always informative" (Cosmecology Bulletin USA). "What a book! It will go into history as one of the most important books of western astrology" (Tijdschrift Astrologie The Netherlands). "A major work, indeed the only one of its kind" (Professor H J Eysenck, University of London).

But that was 40 years ago. Since then there have been new tests, new approaches, and a thousand new studies using computers for astrological calculations previously impossible by hand. In other words a New Understanding is now within reach. But books continue to parade views either for or against based on incomplete surveys of a mostly hard-to-find literature with no nuanced views. So this new survey collects all the evidence -- not just the bits selected to prove a point -- and subjects it to rigorous critical thinking. The results explain why for many people there is more to astrology than being true or false.

Covers of some astrology journals and books by AinO Publications

This new survey evolved via earlier works published in Amsterdam by AinO Publications, long-time publisher of research journals and books for the Nederlandse Vereniging tot Wetenschappelijk Onderzoek van de Astrologie, the Dutch Society for the Scientific Study of Astrology. The Society began in 1971, ten years before the AA founded Correlation, and was perhaps the first in the world to be aimed at both serious practitioners and serious critics while maintaining high standards on both sides.

Shown above, the journals are the Dutch Journal of Astrology started in 1977, Science and Astrology 1981, Astrology under Scrutiny 1986, and the same in English 2000. The books are Astrology under Scrutiny 2013 featuring an anthology of articles since the first issue, which was expanded into Tests of Astrology 2016 to include more international work (the cover illustrates Gauquelin's famous Mars effect). Six more years of work led to the present Understanding Astrology: A critical review of a thousand empirical studies 1900-2020, published in late 2022, which in effect updates the earlier 1900-1976 coverage of Recent Advances. It also involves some of the original compilers and helpers.

At 1.8 kg Understanding Astrology is seriously comprehensive
It has 952 pages 170x240 mm, and more than 650,000 words, 750 pictures, 650 graphs, nearly 500 tables, over 4000 references, glossary (85 entries), name and subject indexes, and a concise style free of waffle. The mean measured readability is well within ordinary education levels, which with an average of just over one bold heading per page makes navigation easy. Some parts are necessarily technical but there are always plain-English summaries helped by hundreds of explanatory visuals like these:

Examples of visuals

Hardcover copies with a sewn binding (lies flat) are available direct from the publisher in Amsterdam for 65 euros each (about $US60 or 6c a page of typically 700 words) excluding air mail postage -- the Dutch post office does not deliver by sea mail. You can pay by PayPal or ebank but not credit card. To check availability and postage costs, please email the publisher wout DOT heukelom AT hetnet DOT nl. The book has an ISBN (978908-249291-0) but to avoid high prices it is not available from bookstores (which would more than double the price). The entire work is Open Access and can be downloaded for free as four searchable pdf files totalling 39.2 MB. Each pdf takes up to three minutes to download depending on your computer and local traffic.

Exterior of Understanding Astrology

To download the pdf files visit the download page. No registration or login is required. You are welcome to use the material and distribute it to others for free but be sure to mention the source (title, authors, publisher, book date, page numbers). The publisher welcomes comments (email is in previous paragraph).

So why buy a hardcover copy?
To avoid huge file sizes the downloadable pdfs are necessarily lower in resolution than the printed version. Text is largely unaffected but fine detail in the pictures and graphs may be hard to read. You are welcome to print out a full copy rather than squint your way through a thousand pages on-screen, but you will of course need lots of paper (print size is already at the lower limit for continuous reading, which makes 2-up copies a challenge). A hardcover copy is cheaper (6c US a page), nicer to look at, lies flat, and is much easier to use with no loose pages to clutter your reading space. As one Swiss reader told us, "I have seen your website, downloaded the four pdfs, and would like to buy a hard copy".

Praise for Understanding Astrology
"A mammoth undertaking ... will soon become the de facto standard reference"
      -- Philip Graves, historian, world's largest astrology reference collection, Wales.
"It will be the classic sourcebook for all previous astrology research"
      -- Brad Kochunas, licensed counsellor, Ohio.
"The ultimate guide, truly comprehensive, one of a kind, promotes critical thinking"
      -- Catherine Sinclair, researcher, Université de Lyon, France.
"Surely THE definitive critical review of astrology research"
      -- Professor Caroline Watt, University of Edinburgh.
"Very impressive tour de force"
      -- Proressor James Alcock, York University, Toronto.
"The definitive work on the topic, will be consulted for years to come"
      -- Professor Richard Wiseman, University of Hertfordshire.
"Monumental accomplishment"
      -- Professor John T Burns, Bethany College, West Virginia.
"I am overwhelmed -- invaluable to anyone interested in astrological research"
      -- Dr Peter Niehenke, former president of the German AA, Switzerland.
"Understanding Astrology is a book that every serious astrologer would do well to tackle and face up to. There is a vast amount of material that presents a serious and as yet unanswered challenge to astrology as an objective science [he is referring to page 22], especially within the context of what it is that astrologers currently do in practice"
      -- Chris Odle, astrologer, 40 years experience, website
"It is a wonderful book! Beautifully laid out, easy to handle, choice of paper is excellent, size of the book is perfect. And the pages lie flat without print being smashed into the center binding. At $US60 it is like giving away a book of such quality. Publishing the book in hardback was an excellent choice. I expect I will be reading it for many weeks, and sections for years. Many thanks for a miraculous publication!"
      -- Therese Hamilton veteran NCGR researcher in California.
"I consider this work to be a true masterpiece and I am planning to spread its message ... the only way for astrology to gain serious consideration is by acknowledging its shortcomings and striving for empirical evidence."       -- Aleix Mercadé Cosmograma School of Astrology (the largest school of astrology in Spain and its most active scientific research group).
"A milestone in bringing together in one place reported studies and related writings on astrological claims that were previously impossibly scattered. The result is a critical synthesis of information well beyond the confines of astrology. For believers, skeptics, historians, academics, and researchers generally, this is a challenging, invaluable and unique resource."
      -- Donald H Saklofske, professor of psychology at the University of Western Ontario, past editor of Personality and Individual Differences, past president of the International Society for the Study of Individual Differences.

Published reviews of Understanding Astrology
To date three reviews have been published. More are coming but will take time.

Cover of Skeptical Intelligencer

The first review is by Ray Ward, a retired librarian, former believer in paranormal matters, and now a book reviewer with a special interest in astronomy and astrology. Unlike most reviewers he was familiar with Recent Advances and was able to revisit it at the British Library. His review in the quarterly Skeptical Intelligencer, published by the UK's Association for Skeptical Enquiry, 26(1), 10, 2023, is very brief (210 words) for reasons he is careful to explain. He concludes:

"In the Skeptical Intelligencer 19(4), 8-9, 2016 I reviewed Tests of Astrology: a Critical Review of Hundreds of Studies by Dean and others (AinO Publications 2016). Now comes this 'revised and enlarged edition'. It is nearly twice the size of its precursor, which I described as 'the most comprehensive summary of the subject', and this one is even more so. There would, however, be little point in attempting a full review, since everything I said about the earlier work applies to this one, which simply strengthens what the earlier work said. Just don't drop it on your toe, or the stars predict a trip to the hospital!" [the book weighs 1.8 kg vs 0.4 kg for a typical paperback].

Ward's earlier review was quite thorough (1480 words). He quotes Recent Advances: "The astrological literature is filled largely with demonstrations of belief. What it is not filled with is demonstrations of truth;" and sees Tests of Astrology as supporting and strengthening this assertion: "It includes personal stories of astrologers and believers who changed their minds; the discovery of astrology; the evolution of tests and why they are needed; a whole section on the Michel Gauquelin work; hundreds of empirical studies over 1927-2015; test overviews; artifacts (including a good general discussion of why people see what they expect to see); the future of astrology; the case for and against; a glossary; and name, subject and book indexes".

He concludes with his own overview: "[Tests of Astrology] ends with a summary prefixed by a quotation from Christopher Hitchens 'What can be asserted without proof can be dismissed without proof': there is no physical way astrology could work; hundreds of tests have shown it does not deliver useful factual truth; the claimed as above, so below links do not exist; charts are meaningful even when wrong; outcomes are explained by hidden persuaders; and claimed experience is unfounded because it is never tested under controlled conditions. Hits are chosen, misses ignored, failures explained away, and unwelcome test results dismissed because, it is said, astrology cannot be tested."

Cover of Skeptical Inquirer The second review is by Terence Hines, professor of psychology at Pace University New York, adjunct professor of neurology at New York Medical College, and author of the acclaimed Pseudoscience and the Paranormal 2nd edition 2003, over 850 references, covering the whole field of paranormal claims and how to evaluate them scientifically. Beware "the constructive nature of memory and related cognitive illusions" (p.147), where the uninformed see only what they want to see. His review appeared in Skeptical Inquirer 47(5), 62-63, 2023. In 1400 words he outlines the book's eleven chapters and their contents. Here are some extracts (490 words):

"Understanding Astrology is a thorough compilation and review of "under the radar" [ie hard-to-find] research, much of which is quite interesting. Its text is divided into eleven chapters. After the introduction, Chapter 2, "Personal Stories," is unusual, entertaining, and informative. ... Chapter 3 is a brief (fourteen-page) history of astrology, including the Chinese and Hindu versions. Chapter 4, "Disagreements in Astrology", covers both disagreements among astrologers and between astrologers and scientists.

"Chapter 5 describes the early (starting in the 1500s) tests of astrology. ... Chapter 6 is devoted to one of the most contentious controversies in modern astrological research, Michel Gauquelin's [Mars effect]. ... The authors argue that the effect is real but not due to astrological influences. ... The argument is both complex and convincing.

"Chapter 7, "Individual Studies," is by far the longest chapter in the book at 556 pages, or 60 percent of the total page count. ... an especially useful characteristic of the book [is that] Studies are not just described but are critiqued. Where authors have made logical, methodological, or statistical errors, these are noted. Some of the studies summarized, as well as some of the authors' critiques, use sophisticated statistical analyses.

"While chapter 7 described and discussed each test of astrology individually, chapter 8, "Overviews," discusses tests by topic, such as tests of sun signs [and] tests of matching horoscopes to personality test results, ... The chapter also covers statistical issues and concludes that there is no evidence that astrology has any validity. Those few tests that seem to support astrological predictions can "be explained by knowledge of serious astrology biasing the results." ... Chapter 9 concerns artifacts that lead to belief in astrology, such as reliance on personal experience, confirmation bias, seeing patterns in random noise, and the representativeness heuristic, among others.

"Chapter 10, "Future of Astrology," is a short chapter of sixteen pages. This is appropriate because the large corpus of studies reviewed in this book show that astrology has no empirical foundation. ... Some astrologers resort to using it as a basis for counseling. But the authors wisely note that "Astrology by itself is not counselling. People with problems need to learn coping skills, but this will not happen unless the astrologer is properly qualified." ... Chapter 11, "The Case for and against Astrology," ... approaches the issue from a broader perspective that asks whether astrology can be seen as a belief system that can function and be beneficial even though it is scientifically invalid.

"To say that Understanding Astrology is comprehensive is an understatement. It covers every aspect of Western astrology that I can imagine ... in clear and concise detail. It took a huge amount of work to compile the information in the book. The coverage is not limited to material in English. The text is extensively illustrated with reproductions of diagrams and figures from the publications discussed. Also delightfully included are copies of relevant book covers, artwork, and amusing cartoons. In short, it is the definitive work on the status of astrology."

Cover of 
Correlation The third review is by Robert Currey, editor of Correlation: The Astrological Association Journal of Research in Astrology, in 35(2), 63-83, 2023. Currey is perhaps best known for his Equinox range of astrological services including chart and phone readings. Unlike reviewers Ray Ward and Terence Hines he finds Understanding Astrology "too detailed to review in one article and so I can offer only a brief overview here" (p.65). But his "brief" 7800 words do not tell readers as much about the book's contents as do Hines's 1400 words. Currey summarises his own review in two places:

First in his editorial (p.6):

"[The authors] make no substantial criticism of important recent research. ... They misreport data and they argue by using strawman and other fallacies. There are no discoveries of artefacts, no issue of lack of control groups, no reporting of statistical errors or under-sized samples ... This unscientific approach is symptomatic of critics in decline who need an easier target than empiricism allows."

Later (pp.80-81) he summarises his review as follows:

"Instead of a critical review the authors present fallacies and misinformation. The thousand viable empirical studies do not exist and while the authors understand the techniques of astrology, they lack a genuine understanding of the practice of astrology."

Readers aware of Understanding Astrology's 4000 references might wonder how a thousand studies could suddenly disappear. And why there is such a marked conflict between Currey's views and those of the other two reviewers (for example they note the detailed treatment of issues that Currey says are ignored as in the first quote above), and also those of the many informed readers listed above in red.

Arguably Currey's review reflects his many years of experience selling his astrological services. But there are too many errors in his quotes from Understanding Astrology for his review to be taken seriously (see below), at least by readers who expect freedom from conflicts of interest when reviewing negative empirical findings.

Readers face the same problems with AA president Roy Gillett's own dismissal of the book in his article "(Mis)understanding astrology" in The Astrological Journal July/August 2023 pp.28-29. He finds no appreciation of "the valuable way genuine astro-cycle study can be used to understand [daily experience]" produced by "background pressure (outer-planetary cycles), the contemporary mood (inner planet cycles), and triggers to act (Moon and angles)," for which readers should see his own books for a "detailed astro-cycle background." Instead Understanding Astrology has only "tedious pages ... failed studies ... nit-picking narratives ... based on sand ... disconnected opinions ... and authors who misunderstand astrology and what astrologers do." That two of the authors are former full-time professional astrologers and teachers of astrology is somehow overlooked, as is the lack of evidence for his claims under controlled conditions. But back to Currey.

Although Currey's erroneous quotes are too numerous to justify the space needed to address them here, many examples are addressed in a six-page handout given as part of the publisher's invited presentation on Understanding Astrology at NVWOA's October 2023 symposium in Zwolle called Astrology and Science (pictured below before the start, see caption). NVWOA is the Dutch Society for the Scientific Study of Astrology founded in 1971 for the critical study of astrology. Airmail copies of the handout are available from the publisher in return for your mailing address and subsequent comments.

NVWOA Symposium

NVWOA president Frank Vernooij at the podium busy with technicalities prior to summoning members for the first talk, which was publisher Wout Heukelom's Understanding Astrology. Hoe kwam het zover? [How did it come to this?]. Photo by Heukelom.

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