Your Zodiac Sign, Your Health

Your birthday is an inescapable mark, one of a small number of qualities in life you can never change, that accompanies you everywhere. Think about how often you write out or speak aloud those eight digits; they are one of the core confirmations of your identity, now and forever. I’m “Elijah Wolfson,” but to many I’m also “09/20/1985.”

The immutability of one’s nativity may be why so many are drawn to astrology—according to a 2009 Harris Poll, a full 26 percent of Americans believe in astrology (less than the number of UFO believers, but more than those that accept reincarnation as truth). Astrology builds on your birthday, giving it a meaning that goes well beyond that arbitrary 24-hour period so many years ago, of which you have no functional memory. It purports to help us understand the world through a system of relationships between astrological phenomena and human experience. Today, it takes its most popular form in the horoscope, found mostly in pop culture magazine and websites, and often focusing on foretelling the reader’s future in the realms of love and money.

But astrology was once a wholly scholarly and scientific endeavor. For example, medical astrology, or “iatromathematics,” stretches as far back as the field of medicine itself. “This is one of the longest standing intellectual traditions on the planet,” says Lauren Kassell, professor at the University of Cambridge’s Department of History and Philosophy of Science. Kassell researches the history of astrological medicine.  “For centuries it was common practice for educated physicians to use astrology as one of their tools,” she says. Hippocrates, whose namesake oath is, to this day, still sworn to by all who enter the field, has been attributed to the saying “a physician without knowledge of astrology has no right to call himself a physician.”

These days, astrology has lost some of its luster, and is generally assumed to be bunk by the scientific community. Their argument: while its lush symbolism and universal symmetry were somewhat effective explanatory models back when we were in the dark, we now can see 13.2 billion light years into space and take picture of things half the width of a hydrogen atom, and it no longer seems particularly wise to choose when to operate based on where Jupiter sits in the sky.

But there’s something to astrology that rationalists have to contend with: The season of your birth, it turns out, appears to have a strong influence on your future. Depending on whether you were born in the spring, summer, fall, or winter, you could have a higher or lower risk for: schizophrenia, multiple sclerosis, sleep disorders, Type 1 diabetes, bipolar disorder, and allergies, among others. The season of your birth also seems to affect how long you live.  

On their own, these individual studies could be the result of abnormal grouping, just random, scattered, clusters. But accumulated, a pattern emerges, begging the question: Is there any science behind the dusty old pickup line “what’s your sign?”

Many contemporary scientists are loath to admit to anything resembling astrology. “It seems absurd that the month you are born/conceived can affect your future life chances,” write neuroscientists Russell G. Foster and Till Roenneberg in a 2008 study. They then go on to then point out no fewer than 24 different health disorders connected to season of birth, and ultimately admit “despite human isolation from season changes in temperature, food, and photoperiod in the industrialized nations, the seasons still appear to have a small, but significant impact upon when individuals are born and many aspects of health.”

The problem may be that there’s no clear underlying mechanism for the observed phenomena. Theories range from levels of maternal vitamin D levels during pregnancy to seasonal viral and bacterial exposure.

“We know that there is this weird connection between seasonal birth and certain disorders, but we don’t know why,” says Chris Ciarleglio, a neuroscientist currently working as a postdoctoral fellow at Brown University. Figuring out the methodology, the “why,” is Ciarleglio’s primary research interest. A few years back, a team of researchers—John Axley, Benjamin Strauss, and Karen Gamble—working under Doug McMahon in the McMahon Lab at Vanderbilt uncovered perhaps the most compelling solution to date—one that, in theory, should please hard-line advocates of the scientific method and astrologers alike. It starts with a clock.

By now it’s popular knowledge that all humans have an internal clock or “circadian rhythm” that regulates our sleep/wake cycle. But this clock does much more than just tell us when to wake up every morning; it is central to all life functions. Cell growth, death, and reproduction; the functioning of digestive tissue, lungs, heart, and liver; patterns of social behavior—it’s all controlled by your biological clock.

If the clock is stable, things go according to plan: your body makes all its appointments. The problem is that no matter how perfectly genetically calibrated your clock, it’ll have to face the outside world, where a slew of environmental factors can permanently (and potentially negatively) alter how well it works. That’s where the season of your birth comes into play.

The McMahon Lab team took a bunch of newborn mice and put them in one of three different environments, each corresponding to a different “photoperiod” of the year: a 12 hour/12 hour split between light and dark (a.k.a. the spring and fall equinoxes), an 18/6 split (the summer solstice), or a 6/18 split (the winter solstice). After 21 days—quite a while in mouse-time—the mice were moved into different photoperiod environments and the researchers looked at how they reacted. Most of the mice adapted to the change just fine, with one exception: The clocks of the “winter”-raised mice became extremely labile—seemingly, in fact, unstable.  

The instability of the winter-raised mind isn't a new concept. Over the years, many studies have shown some correlation between various diseases and disorders and seasons of birth, but one grouping stands out. This particular correlation was found as early as 1929, and more than 200 studies have confirmed the results, including a large meta-study in 2003 accounting for more than 86 million births from 27 different parts of the world. The bottom line: If you were born in the winter, you’re at a higher risk for schizophrenia than those born in the sunnier months.

Similar results were found when researchers studied bipolar disorder and Seasonal Affective Disorder. There’s a reason why.

“Disorders like depression, schizophrenia, or autism—developmental disorders—are linked to the circadian clock,” Ciarleglio says. “The clock is hard-wired to other important areas of the brain that play major roles in how those disorders manifest.”

Light enters the eyes, and a signal is sent to the suprachiasmatic nucleus—the brain’s master clock—which, in turn, then sends signals down raphe nuclei in the brain stem. The main function of the raphe nuclei is to release serotonin to the rest of the brain. Serotonin, of course, helps regulate mood, appetite, and sleep, and an imbalance in the neurotransmitter is the basis of depression. Your biological functions all depend on light.

Traditional astrology was wrong: the moon does not cause insanity. But at the same time, astrology was right: those born in the winter season, when the moon has its longest reign in the skies, are more likely to suffer from mental disorders like schizophrenia and bipolar disorder. Modern science is catching up, and clarifying: Based on Ciarleglio’s and other researchers’ work, we now understand that the lack of sunlight in those months seems to contribute to long-term changes in how genes are expressed. In other words, we are inextricably tied to the specifics of our developmental environments.

In the past, scientists had to take the side of either nature or nurture in an often heated debate. These days, most have agreed that the better answer involves a little of column A and a little of column B. Yes, you are born with a single set of genes that determines many of your innate characteristics, but how those genes are expressed is manipulated by your environment and developmental experience.

“There is this fundamental biological paradigm,” Ciarleglio explains, “where developing in a certain season seems to imprint your circadian clock, and your circadian clock has control over mood and other neurological conditions.”

Counterintuitively, this theory of development and seasonal imprinting was preempted by medical astrology years ago. “It’s not that astrologers thought that you are imprinted by the position of the stars and planets at birth and are then impervious the rest of your life,” Kassell says. While there is a particular set of things that shape you at the time of conception and over time, the environment in which you develop exerts its influence, too—those socio-environmental factors are in fact, according to Kassell, key to what medical astrology looks to uncover.

Ultimately, all systems of medicine are after the same thing: helping patients get and stay healthy, deal with symptoms, and face illness and mortality with the best tools they have. The science of why season of birth matters impacts science. After all, it’s not just occult imaginings—there are real connections between our internal clock and development disorders. As with so many aspects of scientific history, we can look back at medical astrology and recognize that while it may have gotten the mechanisms of action wrong, it got the effects right.  “It’s very useful to look at knowledge of the natural world as something which is produced,” says Kassell. “It’s not something that is found, it’s not a universal truth, and it’s always socially instantiated.” Astrologers in the middle ages produced one kind of knowledge; an MIT laboratory in the 1980s produced another. And they are equally hard to comprehend.