Hot weather has long been linked to violence, but whether or not it is the cause of our species’ bad temperedness remains unclear. In fact, a study conducted at the turn of the 21st century suggested that at around 80 degrees Fahrenheit, assault rates no longer increased with temperature, but actually started to drop. This led some researchers to suggest that perhaps moderate levels of heat, and thus discomfort, provoke aggression, while extreme heat and discomfort made people want to flee, Wired Science reports. Another theory holds that because many displays of violence depend on social opportunity—as the weather increases, and people spend more time outdoors, violent crime rises, but as the temperatures become too hot to handle, and people retreat indoors, the trend stops.
But reanalyzing the data in 2005, Iowa State University psychologist Craig Anderson found that the trend of increased violence with rising temperatures continued even past 80 degrees, if he took account the time of day. Specifically, most assaults occurred between 9 pm and 3 am, when temperatures are generally cooler. And within this time period, Anderson found that higher temperatures indeed led to more assault cases. This interpretation of the data supports physiological hypotheses, such as the increases in heart rate, blood circulation and sweating, as well as metabolic changes, associated with hot weather could alter the activity of the sympathetic nervous system, which may in turn influence a person’s likelihood of escalating a situation or walking away from it. Heat is also known to increase testosterone production, which can promote aggression.
Other hypotheses draw on the notion that physical discomfort has been associated with negative memories, which could alter a person’s reactions towards hostility. Indeed, some studies have shown that overheated people struggle with rational thinking, and hot weather can cause people to interpret neutral situations as hostile or violent. Though these explanations are still entirely speculative, Anderson thinks there may be a real biological explanation for the association found between heat and violence.