Suicide rates don't go up in the winter. They spike in the spring.

Prevailing cultural wisdom tells us that most suicides occur during the cold of winter, during the holiday season when people might be alone or depressed.

But data suggests that isn't right: For the past 15 years, suicide rates have been highest in late spring and summer, although there is evidence this might be changing.

As you can see above, there's a clear drop in the percentage of deaths by suicide in February (but it's important to remember that February has fewer days than all other months) and then a steady uptick starting in late April/early May with a slight dip in June, before rates steadily drop off again in October.

But despite clear patterns that show seasonal variation in suicide rates over time, the role that underlying environmental and demographic factors play in contributing to higher levels of suicide is largely not understood.

The mystery of spiking suicide rates in the summer

Research exploring the seasonal variation in suicide goes as far back as 1825, but there is remarkably little consensus on the factors responsible for triggering the annual spikes in suicide rates.

There are several theories that attempt to explain the seasonal variation in suicide rates — one focuses on the increase in daylight and another on additional pollutants in the air. But few environmental explanations have gained as much traction as the idea that allergens can cause violent, suicidal behavior.

Some researchers have argued that higher rates of suicide correlate with seasonal peaks in ambient pollen concentrations. For instance, in spring there is a high concentration of tree pollen in the air, in summer, grass pollen, and in the fall, ragweed. Several studies have linked the inflammatory reactions triggered by allergens to increased anxiety, reduced social interaction, and aggressiveness, all of which are potential warning signs of suicidal behavior.

Still, this research remains somewhat inconclusive, and far from a definitive answer on why suicide rates increase in the spring, as the relationship between seasonal allergens and its role in exacerbating mental states is not fully understood.

For heavily urbanized areas, a second peak in suicide rates emerges in the fall

While the exact underlying reasons for why suicide rates increase during spring are disputed, the majority of research does agree on one thing: Rural areas are disproportionately affected by spring suicides compared with their urban counterparts.

Researchers have found that predominantly agricultural countries have a higher rate of spring suicides than industrial countries, and intense outdoor labor correlates with a greater spring peak in suicide. These findings have led many researchers to conclude that seasonal factors like day length, temperature, and allergens may be more damaging in a predominantly outdoor environment given their increased prevalence.

But what's particularly interesting about this theory of a rural/urban divide in suicide seasonality can be seen in new research that focuses on what seasonal suicide rates look like in predominantly urban areas.

Some researchers, such as Ajdacic-Gross et al., have argued that as we shift from primarily rural economies to highly industrialized societies, seasonal variation in suicide rates flatten over time. But researcher Simo Näyhä found that instead of seasonal variation smoothing out, a second peak in autumn is emerging for people who work in more modern office jobs.

And this observation definitely appears to be true for the US in recent years. In September 2014, the US recorded its highest number of suicide deaths of any month in the past 15 years, with 4,025 reported suicides. Additionally, the rate of suicides in September has steadily crept upward since 2010.

Women have two seasonal peaks in suicide rates, and men have one. But no one knows why.

Multiple studies have shown that women experience two peak seasons of suicide rates, in the spring and fall, while men only have one peak, in the spring. Trying to answer why women experience two peaks in suicide rates when men only experience one is, at this stage, largely speculative, however.

Researchers understand that a difference in seasonal variation exists between the sexes but are at a loss as to what causes this difference.

There is one study in England that found that middle-aged women who had school-age children were more likely to die by suicide in fall (or the beginning of the school year), which tentatively suggests that reduced contact with dependents could possibly represent a type of suicide risk in women.

Additionally, the latest Centers for Disease Control and Prevention report on suicide rates found that women experienced a greater percentage increase in the number of suicides from 1999 to 2014 than men (even though men still outnumber women in total number of deaths by suicide).

And while it still doesn't explain why, an examination of the monthly data shows that significant percentage increases in suicide rates were heavily concentrated in the autumnal months for women.

If you are considering suicide, please seek help through the national suicide prevention lifeline at 1-800-273-8255.