Season of Birth Research: Can it really be used to prove astrology?

Numerous studies have shown a link between season of birth and personality, mood swings, age when babies crawl and more. But before getting too excited, we should consider what these studies showing a seasonal biological effect are measuring and what the implications might be for astrology.

A 2010 study "provides the first evidence for seasonal imprinting of biological clocks in mammals." Using baby mice, the researchers showed that the imprinting at birth had "dramatic effects on the reaction of the biological clock to changes in season later in life." But the imprinting was not specific to the actual time of birth. "Exactly when the imprinting occurs during the three-week period leading up to weaning and whether the effect is temporary or permanent are questions the scientists intend to address in future experiments."

Other studies have shown that there is a link between our biological clock and diabetes, that fiddling with our biological clock can suppress cancer, that babies born in the winter and early spring have a higher risk of schizophrenia, and that the month of birth can affect allergies. The issue for astrologers is that these differences are specific to latitude. In other words, January in the northern hemisphere shows the same results as July in the southern hemisphere. And it appears that these differences do not show up on the equator.

So, does this type of research really provide positive evidence for Sun sign astrology? If so, does this mean we need to modify how we practice astrology to better account for seasonal variations by latitude?

Research Studies on Season of Birth

Birth season linked to risk of later-life mood disorders (2014).
Research from Hungary indicates that season of birth may be related to difficulty with moods (such as seasonal affective disorder or SAD). Generally, research found that people born in the spring or summer were more positive and cheerful than those born in fall or winter. 

Age which babies crawl (2014)
According to the American Pregnancy Association, another difference that appears depending on the season is that winter babies crawl sooner than summer babies.

Astrological article discussing implications of "season of birth" research
The Pineal Gland And The Ancient Art Of Iatromathematica by Frank McGillion (dated unknown - possibly 2000)
The medical astrologers of Ancient Greece: the iatromathematici, and the later European physician-astrologers, assumed a correlation between events in the heavens and those on earth that was relevant to both health and disease. Some of the early practitioners of modern scientific medicine did the same under the aegis of what we might term, proto-cosmobiology, though none of them could provide an adequate mechanism to explain the nature of the link they believed existed between the skies and ourselves. With the discovery and elucidation of the pineal gland’s functions in the mid twentieth century, which are discussed in detail, we were in a position to provide such a link and we can now, to a great extent, explain in conventional scientific terms, how those influences of the sun, moon, planets and other celestial phenomena studied by the early iatromathematici and early cosmobiologists could, can, and do, affect us.

Below is a list of additional research articles

Season of birth may have long-term effects on personality (Dec 2010)
The season in which babies are born can have a dramatic and persistent effect on how their biological clocks function. (research done in United States)

Season of birth may affect kid's allergies. Oct 19, 2010
Researchers say the incidence of an allergic response to certain foods varied according to season of birth, ranging from 5% for infants born in June and July to 9.5% for those entering the world in October and November. This research was done in Finland.

Association of autistic spectrum disorder with season of birth and conception in a UK cohort. (abstract only)
A total of 86 children with ASD were identified in the ALSPAC cohort giving a prevalence of ASD of 61.9 per 10,000. There was some evidence for an excess of children with ASD being conceived during the summer months with a rate per 1,000 conceptions of 9.5 in summer compared to 5.1, 4.6, 5.7 in spring, autumn and winter, respectively. A doubling of the odds was suggested for summer compared to autumn (Odds ratio 2.08 [1.18, 3.70]). In agreement with previous research, there was a corresponding peak in spring births. Conclusion: Conception during the summer months was associated with an over-representation of children with ASD in this UK birth cohort. There was also an association between ASD and spring births. Further investigation of seasonal influences on the aetiology of autism is required to identify possible factors in the environment, and their mechanisms and timings.

Seasonality in birth weight (2009)
Seasonality in birth weight: review of global patterns and potential causes. Published August 2009 (abstract only)
In this review we summarize the literature on seasonal variations in birth weight. Although causes of seasonal variation in developing regions are more clearly understood, it is not yet clear which factors affect apparent seasonal variation in birth weight in developed countries. In our analysis we observed a pattern of seasonal variations in developed countries that differed between low-, middle-, and high-latitude countries, and we suggest several mechanisms that may be responsible for this diversity. Namely, we suggest that in middle-latitude climates, the large annual temperature range may cause low birth weights during summer, whereas in high- and low-latitude regions variations in sunlight exposure between seasons may contribute to low birth weights apparent during winter.

Season of Birth and Later Outcomes: Old Questions, New Answers. (2008) (PDF of paper)
Research has found that season of birth is associated with later health and professional outcomes; what drives this association remains unclear. In this paper we consider a new explanation: that children born at different times in the year are conceived by women with different socioeconomic characteristics. (Research done in the United States.)

Month of birth and offspring count of women: data from the Southern hemisphere (2008) (abstract only)
We find that the association between birth month and offspring count of New Zealand women born in the Southern, albeit not Northern, hemisphere is a mirror image of the pattern reported from Austrian women: on average, women born during the Southern hemisphere summer months have fewer children than women born in winter. This association is highly significant within the lowest family income category but insignificant within higher family income categories.

A Birth-Season/DRD4 Gene Interaction Predicts Weight Gain and Obesity in Women with Seasonal Affective Disorder: A Seasonal Thrifty Phenotype Hypothesis (2007) (article)
As reviewed by Torrey et al (1997), research into the effects of birth season on psychiatric illness dates back to the 1920s, and was based on the general hypothesis that seasonal changes in diet, vitamins, and/or sunlight would affect early brain development and thus later psychopathology (Tramer, 1929). By the 1960s, several large systematic studies were initiated, particularly in schizophrenic and manic-depressive psychosis, and before the turn of the millennium over 250 such studies had been completed in over 400,000 individuals. Of the 19 largest Northern Hemisphere studies on birth season and schizophrenia, all but one reported a statistically significant excess of winter–spring births compared to a group of matched controls. A similar pattern has been found in manic-depressive psychosis, although with less consistency across studies (Hare and Price, 1968; Dalén, 1975). Preliminary work in nonpsychotic bipolar disorder also points to a winter–spring birth excess (eg Torrey et al, 1996). . . . The current results extend our genetic model of weight gain and obesity in women with SAD by suggesting that birth-season interacts with the DRD4 gene to influence body weight regulation in this population. (Research done in United States)

Season of Birth and Dopamine Receptor Gene Associations with Impulsivity, Sensation Seeking and Reproductive Behaviors (2007) (abstract)
These results are consistent with past findings that SOB (season of birth) is related to sensation seeking. Additionally, these results provide tentative support for the hypothesis that SOB modifies the behavioral expression of dopaminergic genetic polymorphism. These findings suggest that SOB should be included in future studies of risky behaviors and behavioral genetic studies of the dopamine system. (Research done in United States.)

Why season of birth is related to childhood intelligence (2006) (review) (abstract)
Countless studies have found that children’s intelligence appears to be related to the time of year they were born in. Some investigators have argued this is because seasonally varying environmental factors like temperature and infections can affect brain development. But now Debbie Lawlor and colleagues have analysed data from 12,150 children born in Aberdeen between 1950 and 1956, and they’ve concluded that the effect of season of birth is almost entirely explained by the age children happen to be when they start school.  (Research done in United States.)

Related articles on Schizophrenia

Season of Birth - Low Sunlight Exposure/Vitamin D deficiency is associated with higher risk of schizophrenia
According to an article in the New Scientist magazine, research suggests people who develop schizophrenia in Europe and North America are more likely to be born in the winter and early spring (February and March in the Northern Hemisphere) In other words, the subjects who were born during these months had a slightly higher than average rate of schizophrenia, while subjects born in August and September had a slightly lower than average rate. There seems to be about a 10% difference in risk of schizophrenia between the high (Winter and Spring) and low risk months of birth.
One possible reason that researchers believe may explain this seasonality of schizophrenia risk is the association between winter/spring births and schizophrenia may be related to sunlight exposure. A lack of sunlight (for example, during the shorter days of winter) can lead to vitamin D deficiency, which scientists believe could alter the development of a child's brain in the mother's womb and after birth.

Season of birth in Japanese patients with schizophrenia. (April 2002 abstract)

 A number of North American and European studies have observed a higher proportion of winter births in schizophrenia patients. Fewer studies have investigated this issue in Asian populations, and the findings are not as consistent as in the studies of Western populations. A statistically significant excess of winter births has not been observed in Japanese or Korean studies, while some of the studies found a decreased number of summer births among their patients. We further investigated the issue in Japanese patients with schizophrenia (n=2985). No significant excess of winter births was observed. Spearman correlation between schizophrenia births and ambient temperature was not significant. However, a decrease in the summer births was found, consistent with most of the previous Japanese studies. When analyzed by gender, a decrease in summer births was found in males, but not in females.

Long-term trends in sunshine duration and its association with schizophrenia birth rates and age at first registration--data from Australia and the Netherlands. (April 2002 abstract)

 Both the Dutch and Australian data showed a statistically significant association between falling long-term trends in sunshine duration around the time of birth and rising schizophrenia birth rates for males only. In both the Dutch and Australian data there were significant associations between earlier age of first registration and reduced long-term trends in sunshine duration around the time of birth for both males and females.

A systematic review and meta-analysis of Northern Hemisphere season of birth studies in schizophrenia. (2003 Abstract.)

 Based on the epidemiological finding that individuals with schizophrenia tend to be born in winter/spring when compared to the general population, we examined (1) the strength and timing of this effect in Northern Hemisphere sites, and (2) the correlation between the season of birth effect size and latitude. There was a small but significant positive correlation between the odds ratios for the season of birth comparison and latitude (r = 0.271,p < 0.005). Furthermore, the shape of the seasonality in schizophrenia births varied by latitude band. These variations may encourage researchers to generate candidate seasonally fluctuating exposures.

Season of birth in schizophrenia: no latitude at the equator. (2000 article)

 Conclusions In an equatorial region, where ‘seasons’ are absent, no seasonal excess in births of those later developing schizophrenia was evident. (Research done in Singapore)