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Understanding Astrology:
A critical review of a thousand empirical studies

Abstract -- Fifty years ago astrologers could cite a handful of tests to support their claims. Today there are new tests, new approaches, new data, and a thousand new studies, all of them described in this new book containing information not available elsewhere -- not in astrology books, not in science books, not even on this website. With more than 600,000 words, 1500 visuals, 2000 references, and a concise style free of waffle, it is a unique key resource for anyone curious about astrology. The entire work is also Open Access. When published later in 2020, printed copies (896 pages) will be available direct from the publisher or you can download one for free from this website. See full article for details.

Cover of Recent Advances The 50-year evolution of Understanding Astrology
Fifty years ago astrologers could cite a handful of tests to support their claims. Examples include John Nelson on shortwave radio quality, Vernon Clark on the predictive ability of astrologers, and Michel Gauquelin on professional occupations. These tests were reviewed in Recent Advances in Natal Astrology: A Critical Review 1900-1976, which was the result of a collaboration between more than 50 astrologers and scientists.

Recent Advances was widely acclaimed as the world's best survey of evidence-based astrology. It was "The most important book ever written on astrology" (Phenomena Publications, Canada), "The most talked-about astrology book ever" (Emergence bookshop UK), "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, new data, and hundreds of new studies using computers for astrological calculations previously impossible by hand. Judgement Day has come but with no comprehensive survey until now.
Covers of Understanding Astrology and its precursors The survey evolved via the 2 works (left) into Understanding Astrology: A critical review of a thousand empirical studies 1900-2020, which in effect was an update of Recent Advances's 1900-1976 coverage with more on testing than natal astrology.

All three works are published by AinO Publications in Amsterdam.

Understanding Astrology is seriously comprehensive
It occupies 896 B5 pages (176x250mm) and has a laminated soft cover with a durable sewn binding that opens flat. It contains more than 600,000 words, 1500 visuals and tables, 2000 references, glossary, name & subject indexes, all of it concise and free of waffle. Average grade level for 90% comprehension is US college entry. Frequent headings averaging 1.1 per page make navigation easy. ISBN 978-90-824929-1-0.

How to get a free copy
The entire work is Open Access. When published later this year, printed copies will be available direct from the publisher or you can download one for free as several large pdf files from this website. Of course a downloaded copy may be free but for most people printing out hundreds of loose pages is not. So buying a nicely-bound printed copy from the publisher could save you money and much bother. Price will be about 25 euros (roughly £23 or $US28), not including air mail postage -- the Dutch post office does not deliver by sea mail. Because the book is non-profit (neither compilers nor publisher make money from it) it is not available in bookshops despite its ISBN. You can pay by PayPal or ebank but not credit card. For how to pay via these options and to check book availability, price and postage costs, email the publisher wout DOT heukelom AT hetnet DOT nl.

Why should anyone be interested in Understanding Astrology?
Depending on who we are, astrology can be entertaining, beautiful, dangerous, lucrative, or simply stupid. But always challenging, because half the population believes in it, skeptics deny it, vested interests distort it, astrologers disagree on everything, and most scientists are not interested. Something this popular and this confused deserves critical scrutiny based on facts -- except facts have always been the hardest things to find in astrology. Until now. And that is not all, for Understanding Astrology comes with this unexpected bonus:

Wide interdisciplinary appeal
Understanding Astrology critically reviews over 800 empirical studies from a hundred different journals, plus many others on related topics such as divination, cognitive artifacts, and effect sizes, to which it adds dozens of books, theses, websites and conference reports. The reviews average 700 words and are unusually detailed, so with luck you won't need access to the original documents. About half the reviews submit the data to further analysis, for example by factor analysis or meta-analysis (a thoroughness not found elsewhere). The conclusion is that, superficially, astrology is just a cover for the operation of non-astrological factors that better explain the outcomes. But the devil is in the detail, which involves wide interests over many disciplines. In other words there is more to it than just astrology. For example much will be useful when planning research into cultural influences or when exploring popular areas of belief, or as examples in courses on critical thinking or research design. You can check this for yourself via the book's contents listed later on this page.

Group picture Yes, it's been a long time coming
Planning began in May 2008 in Utrecht (pictured) and continued for twelve years while material obtained from university libraries, academic databases and astrology collections around the world was being reviewed, and important gaps were being filled by in-house testing. It was a long hard slog. As far as we know, nothing important has been missed.

The production team consisted of (from left): Dr Geoffrey Dean (a Brit based in Australia, the next three are Dutch). Rudolf Smit (author in 1976 of De Planeten Spreken, the planets speak, a former editor of the astrological research journal Correlation and founder in 2000 of this website, now recognised as the leading website for the scientific study of astrology). AinO publishers Cygnea van der Hooning and Wout Heukelom. Top: Arthur Mather (Scotland, co-compiler in 1977 with Geoffrey Dean and 52 collaborators of Recent Advances in Natal Astrology, rated by the astrology-related journal Phenomena as "the most important book ever written on astrology"). Bottom: Dr David Nias (England, co-author in 1982 with Professor Hans Eysenck of Astrology Science or Superstition, the world's first book on astrology written by psychologists for ordinary readers and the standard reference in academic studies; it has several foreign-language editions).

Publisher logo Astrologers usually dismiss awkward scientific findings by claiming scientists are ignorant about astrology. But this does not apply here because both Dean and Smit are former award-winning professional astrologers. It means Understanding Astrology submits astrology to informed scrutiny from both sides.

Furthermore the publisher AinO Publications did not arrive in the last shower of rain. It began in 1977 as the initiative behind the world's first peer-reviewed journal for scholarly scientific and philosophical studies in astrology (AinO stands for Astrologie in Onderzoek, or Astrology under Scrutiny). It later expanded into book-length works of which Understanding Astrology is the most complete.

CONTENTS of Understanding Astrology
Per page there are typically 600 words, two visuals (pictures, graphs, tables), and at least one heading.
Open it anywhere and you can easily tell where you are and what the topic is. Indexes are manually compiled.
Much is technical but the best non-technical parts are identified for non-technical readers.

About this book
The world of astrology books
Popularity of astrology
Challenges posed by astrology
Science as seen by astrologers
Astrology as seen by scientists
Verdict of a thousand studies

Evidence changed our minds

According to astrologers
Greece. India. China
Too many combinations
Armchair tests
Likely effect sizes

On astrology
On experience & need for truth
On higher realities
On scientific status
On how astrology works
On interpretation
On symbolism
On overcoming disagreement
On books like this

In the beginning
1800s Astro-meteorology
1850s Two examples
1900s Seven examples
1950s Four examples
1970s First critical review

1980s Computers, meta-analysis
1990s Interest in tests declines
Future prospects

Introduction. Early tests
Mars effect
Reactions to Mars effect
Skeptics behaving badly
Independent replications
Heredity. Tributes
Puzzles. Searching for clues
Evidence for social effects
Other factors
Conclusion. Readers' comments
G effect size references

Studies involving signs (N=76)
Application issues (N=114)
Conferences & Surveys (N=37)
Events, tests of (N=66)
Gauquelin topics (N=38)
Personality, tests of (N=75)
Research issues (N=104)
Summary (total N=510)

Problems of individual studies
Effect sizes and meta-analysis
Tests of signs (mostly sun signs)
Tests of other chart factors
Tests of geophysical factors
Tests of astrologers
Tests of time twins
Tests of prediction
Tests of horary astrology
Tests of mind-related factors

Tests of divination
Tests of wrong charts
Verdict. Effect size references

Blame evolution
Seeing patterns in noise
Seeing patterns in birth charts
Hidden persuaders
How to convince clients
Make the chart fit
Make the client fit
Prevent disconfirmation

Need for evidence
Use in entertainment, creative
  arts, history, and counselling
Research on counselling
An honest future for astrology
Using astrology's true nature
A parallel with art therapy
Could astrology be harmful?

Impartiality is elusive
Comparison with similar areas
Hard facts vs personal meaning
Case for astrology
Case against astrology
Why should astrology be
  part of our future?
Case for and against
  astrology as divination
Forming a personal view

Glossary (50 entries)
Name and Subject indexes

What reviewers said about Tests of Astrology
Tests of Astrology was essentially a less comprehensive version of Understanding Astrology.

Review by Tim Trachet, Skepter (Dutch skeptic journal), October 2016 (translated excerpts):
The bulk of this thick book consists of summaries of the studies that have been carried out on astrology in the course of time, altogether more than three hundred publications, in which the interested reader will find a wealth of information. The final chapters are particularly instructive for all who want to have a critical view of astrology. It discusses in detail the many psychological means by which an astrologer can convince his client - cold reading, the Barnum effect, but there are many more. And some pointed observations about the failure of astrologers. The authors conclude that there is more to astrology than being true or false. They give a number of arguments pro and con. For skeptics, the book contains a lot of useful information.

Review by Ray Ward, The Skeptical Intelligencer [UK] Volume 19(4), 8-9, Winter 2016 (excerpts):
Recent Advances in Natal Astrology (1977), perhaps the most frequently referred-to critical work on astrology, is now out of date and this book constitutes the most comprehensive summary of the subject. 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 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. The book ends with a summary: 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.

Unsolicited comments from readers of Tests of Astrology
From the first nine reader responses:
"Very impressive ... Prodigious effort ... Should be the definitive account of astrology for the foreseeable future ... A high standard of professionalism in layout and editing ... An absolute boon to have such a huge amount of information collected in one source ... A perfect reference ... Impressive ... It is TERRIFIC, an astounding reference book ... Congrats on another magnum opus!"

From long-time US researcher Therese Hamilton:
"I am greatly enjoying Tests of Astrology. The book is very attractively laid out, with an initial approach that is entertaining, humorous and factual. The background research required for the thousands of quotes and citations (articles, PhD theses, Internet links, personal communications, 600+ books) can only be called massive. Even if Tests of Astrology provides little or no support for astrology, I see this book as a 'must have' reference for serious researchers. At almost 500 pages of meticulously formatted text and graphics, TOA is actually several books bound into one volume. Congratulations on a Herculean task!"

What reviewers said about Astrology under Scrutiny
This was essentially the final issue (in English) of Astrologie in Onderzoek subtitled Close encounters with science. It included summaries of the best 110 articles from 27 years of Dutch research into astrology.

"The most informed and informative book of its kind -- Correlation 2014.
"Future comments on astrology will hardly be possible without it" -- Zeitschrift fur Anomalistik 2013.
"Remarkable ... will be a staple for the indefinite future" -- D&J Parker, authors of The Compleat Astrologer

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