Abstract -- Three recent studies (Sachs 1998, Castille 2004, Voas 2007) with a combined sample size of 27 million couples have failed to find the slightest evidence for sun sign effects, thus confirming the results of earlier studies (which are briefly reviewed). Studies of sun sign compatibility largely avoid the problems that plague the testing of individual signs, namely those due to demography (depending on place and country some months have more births than others) and astronomy (due to the Earth's elliptical orbit some signs have more days than others), which here generally cancel out. We follow the tests in some detail to see how huge samples can tease out apparent astrological effects only to find them explained by recording bias and other glitches in the data. Despite the giant magnifying glass of huge samples, no sun sign effects could be detected. Interestingly, many studies have shown that sun sign self-attribution has a tiny effect size typically around 0.08, showing that some people have sufficient belief in sun signs to shift their self-image in the believed direction, albeit only slightly. But the same belief seems to have no effect when picking a partner (effect size <0.000,001), showing that such beliefs may shift their own self-image but not the actual behaviour of others. That is, even for them, the real world always prevails over astrology world. In short, lonely hearts (and anybody else including astrologers) who worry about sun signs (and by extension astrology itself) are absolutely wasting their time.
Married couples tend to be similar in many ways including age, height, education, social class, and attractiveness. The effect sizes can be substantial, for example bright men prefer bright women (r = 0.5) and tall men prefer tall women (r = 0.3). In addition, sun sign books, websites, and women's magazines insist that sun signs are just as important for successful romance. For example Judy Hall (Sun Signs for Lovers: The Astrological Guide to Love, Sex and Relationships, Octopus 2005) says "Sun signs can reveal whether people are passionate or romantic, faithful or flirtatious; and whether they are looking for passion or friendship, fidelity or freedom" (page 7). Hall is a professional astrologer, counsellor, and tutor at the London School of Astrology.
Traditionally the harmonious aspects between sun signs (conjunctions, sextiles, trines) are favourable while inharmonious aspects (squares, oppositions) are unfavourable. For example Linda Goodman (Love Signs, Pan-Macmillan 1980) says "when you find a trined, sextiled or conjuncted relationship between two SunSigns ... love then takes on a deeper dimension" (page 3). Sometimes astrological authors rate pairings on scales of 1-5 or 1-10, which can then be summarised in numerical form as in the plots below, where the predominance of conjunctions (0), sextiles (60) and trines (120) is clearly visible.
As shown by the above plots, astrologers can disagree on particular sun sign pairings, for example King and McKinley (like Ptolemy, see below) rate the opposition (180) as largely unfavourable, Hall and Mitz rate it as average, and the other two rate it as largely favourable. Similarly Hall rates the inconjunct (150) as moderately favourable, contrary to the others including Ptolemy ("if the luminaries be in trine or sextile to each other, the cohabitation will most usually be lasting; but if ... in signs inconjunct, or in opposition, or in quartile , the cohabitation will be speedily dissolved upon slight causes, and the total separation of the parties will ensue", Tetrabiblos, Book 4, Chapter 5, Ashmand translation).
Similarly, as shown in the appendix to History, controversies, validity on this website under Sun Signs, Russell Grant and Mystic Meg disagree completely on the compatibility of Gemini-Virgo (90, "sexual incompatibility" vs "sex style matches perfectly"), and similarly on Virgo-Aries (150), Virgo-Pisces (180), Libra-Libra (0) and Aquarius-Libra (120).
Astrologers can also disagree with themselves when their rating of X-Y differs from their rating of Y-X. Thus in the above plots only King, Petulengro and Mitz show no internal disagreement, whereas the ratings by McKinley, Odle, and Hall disagree with themselves on respectively 15%, 22% and 47% of the possible 144 pairings, as when Hall rates Aquarius-Aries (60), Gemini-Virgo (90) and Cancer-Libra (90) as favourable, but not favourable the other way round.
Nevertheless, the agreement among astrologers is still good enough to ensure that sun sign compatibility (if it exists) should be detectable in groups of married couples. For example the mean correlation between the above plots is r = 0.61 (df=10, p=0.04) the individual correlations being as follows:
King Odle Petul Hall McKin Mitz Mean
Here the most agreeing astrologers are Odle and Petulengro (r=0.80) and the least agreeing are Hall and McKinley (r=0.32).
Compatibility is good for testing
* Boning H et al (1978). Results of Research Group on Marriage Partners,
Tijdschrift Astrologie 2.2, 2.
A recent Swiss study
However, a check of his original results shows that his cutoff level for significance is about p = 0.13, contrary to his claim of p = 0.05 (page 44), which means that in each case 144 x 0.13 = 18.7 pairings are expected to be individually significant by chance alone. Overall there are 25 + 13 = 38 individually significant pairings vs 2 x 18.7 = 37.4 expected by chance alone, which means that Sachs's results actually provide no support whatever for sun sign effects.
Furthermore, as noted by Suitbert Ertel in Sach's Astrology File on this website under Sun Signs, the sign preferences of husbands should tend to agree with the sign preferences of wives, but the mean rank correlation between preferences over the 66 unduplicated pairings AR-AR, AR-TA, ... SG-PI, AQ-PI was a negligible 0.075 (p=0.53) for marriages and -0.12 (p=0.31) for divorces, the last being in the wrong direction. (Ertel's rankings used preferences based on Sach's published expectancies, which are rounded and occasionally lead to ties. When Ertel's test is repeated with unrounded expectancies to avoid ties, the mean rank correlations are 0.136 (p=0.28) and 0.073 (p=0.56), respectively. The differences are small and do not affect the outcome.) Ertel concluded there was "no hint of the correspondences we would expect if Sach's results were genuinely due to astrology". More obviously, as shown in the figure below, the deviations from expectancy do not match the predictions of astrology.
A more recent French study
The above results show a small consistent deficit of squares (90) but no consistent excess of sextiles (60) or trines (120). The apparently large excess of same-sign couples (shown in red) appears large only because the scale is small; thus the corresponding effect size is a tiny 0.0039. So this enormous sample offers no useful support whatever for sun sign effects. The excess of same-birthday and same-sign couples is highly significant, but according to Castille an explanation "is yet to be found" (page 30).
A definitive British study
Voas also notes that more than a hundred standard tables summarising the census results have been published. If they do not cover a particular interest, special tables can be commissioned including any of the data collected, subject to the usual considerations of data protection. Once a table has been created it becomes publicly available. In this case a table showing the pairings by sun sign of 10,317,649 married couples in England and Wales 2001 had already been commissioned by someone describing himself as "a philosopher with a degree in statistics" who was "investigating the truth behind astrology".
The table was in general agreement with the results of Castille. It appeared to show a small but significant tendency for people to marry partners of the same sign (about 22,100 more than the 861,800 expected), and a smaller tendency to marry adjacent signs, but no clear conformity to what astrology predicts. Nevertheless the significant excess of same-sign couples seemed large enough to justify investigation. So Voas commissioned a full breakdown of husbands and wives by day and month of birth (366 days x 366 days x four subgroups by husband's age, or nearly 670,000 combinations). As shown in the figure below, this new data showed an apparent month effect that was slightly larger than the apparent sign effect. So both effects were largely due to couples with matching birthdays.
Voas notes that, while some people may be drawn to each other because they have the same birthday, the excess of same-birthday couples is probably largely due to response error. Census forms are typically completed by one member of the household, who through carelessness or forgetfulness may enter their own birthday for that of their spouse. Such errors are supported by a census validation study that found 10,900 couples where one spouse had assigned their own sex to the other. If people can make mistakes about their spouse's sex they can also make mistakes about birthdays.
Indeed, the most common birthday was 1 January (2560 vs 77 expected) with 1 July (563) in second place, indicating that some people entered 1 January (or 1 July) if an exact birthday was not known, a practice known to occur in old people's homes and for people born overseas. Recall that Castille observed the same in French data, and the same is visible in the previous plot of Sachs's data. (There were even more opportunities for the adjustment of birth dates in the 18th and 19th centuries, see The Gauquelin work 2. Opinions, artifacts, puzzles on this website under Gauquelin.)
Furthermore, more spouses than expected were listed as sharing the same day of birth (but a different month) or the same month of birth (but a different day) as the other spouse. This is not necessarily relevant to the analysis of sun signs, although it would be relevant to Castille's day counting, but it does illustrate the way in which recording errors can occur.
A crucial disentangling test
The results were conclusive. Same-sign-different-month couples were not significantly more numerous than expected by chance, whereas different-sign-same-month couples were significantly in excess. Voas concluded that sun signs played no role in the observed excess of same-month couples.
But what about the slightly-higher-than-expected counts for adjacent signs or months? Here Voas explains a census procedure called "imputation". When the census office encounters illegible or missing dates of birth (which account for about 0.5% of all responses), the day is entered as the 1st of the month, and the months are assigned in rotation. Thus if the dates for both spouses were missing, one would be entered as 1st of month M and the other (usually the wife because they are typically listed on the form after husbands) as 1st of month M+1. Such cases will therefore usually assign wives to the month or sign following their husbands, which is precisely what is observed in the previous plots including Castille's. Because of the tendency to give 1 January as a birthday (which date Castille had therefore removed from his data prior to analysis), both the same-sign/month effect and the following-sign/month effect (ie the value of observed/expected frequency) should be highest for Capricorn/January. And it is:
Capricorn same-sign = 1.061, following-sign = 1.008
Results for cleaned data
The excess of same-sign marriages is now less than 0.2% (p=0.08), no more than would be expected by chance, and much less than the previous 2.7% (p<10-100), confirming the bias due to recording errors and the processing of missing data. Overall there are no variations more than expected by chance, as shown below for the 144 sign pairings.
Interestingly, sun sign self-attribution has a tiny effect size typically around 0.08 (see Meta-analyses on this website under Doing Scientific Research), showing that some people have sufficient belief in sun signs to shift their self-image in the believed direction, albeit only slightly. So we might expect the same belief to lead such people to consider sun signs when picking a partner. That no effect can be detected even with enormous samples shows that their beliefs may shift their own self-image but not the actual behaviour of others. That is, even for them, the real world always prevails over astrology world.
"Astrologers are likely to complain that full birth charts are needed to predict personality accurately. There are two responses. First, to the extent that astrology has influenced everyday belief, it is almost entirely through the use of sun signs; if those were seen as useless when it comes to assessing personality and romantic compatibility, then astrology would lose its hold on the public imagination. Secondly, the basic signs are important even in professional charts. If they had any direct influence, however small, the giant magnifying glass of this huge sample would reveal it. No effects can be detected.
"The only remaining defence is that sun signs mean nothing on their own. [However] such a claim would mean that no generalizations about sun signs could be true. Virtually all astrologers, however, make statements about the supposed influence of different elements of a chart, including the sun sign."
In short, lonely hearts (and anybody else including astrologers) who worry about sun signs (and by extension astrology itself) are absolutely wasting their time. Nevertheless sun sign astrology is unlikely to go away, simply because there is a huge commercial profit to be made from its continued existence, along with wide ignorance of how to judge its claims.