It's a question often asked when a man mistreats a woman: What does she see in him?

A study out of Germany sheds light on why aggressive males can be so appealing. It all begins with the vexing process of mating. According to Parental Investment Theory, reproduction is much more costly for females than it is for males. While women invest nine months in a , a man's initial contribution to boils down to just a few minutes. Thus, women need to be particularly selective when it comes to choosing a mate. Aside from a man’s resources, his genetic gifts are decisive in the selection process. After all, in our ancestors' unpredictable environment, a hardy constitution went a long way in fulfilling evolution's ultimate aims: survival and reproduction.

Research has established that, generally speaking, women must choose between two types of men: dads and cads. On the one hand, dads are typically more commitment-oriented, warm, faithful, and reliable. Yet they are usually less handsome, , and dominant than his caddish counterparts. On the other hand, cads are sexier, with their narrow eyes and strong jaws—but they also tend to be flashy and exploitative of others. Even worse, these masculine men often embody the Dark Triad, a constellation that encompasses , , and . So, what in the world is appealing about these objectionable individuals? Quite simply, they possess high-quality that they will pass down to their future children. In turn, the thinking goes, women will maximize their reproductive success by choosing a macho man as a short-term mate for his genes, and a less masculine man with a warmer personality for a long-term, invested partner.

However, ovulation can make the choice between dads and cads particularly challenging. Research has revealed that during ovulation, women show a weakness for masculine men with high-quality genes. Studies consistently show that fertile women prefer men who display macho facial features and social dominance. In other words, they like bad boys. Building on these findings, Gilda Biebel of the University of Konstanz and her colleagues reasoned that evolution may have also favored men who went to war. Consequently, they wondered if might also be a signal of genetic fitness.

Aggressive behavior comes in two forms. The first is “reactive–impulsive,” which are responses to external threats. The second is “appetitive-aggressive,” which is internally motivated. It is derived from the intrinsic pleasure that is associated with violence, hunting, and combat. In keeping with studies that have found women's preferences for “bad boys” and socially dominant men for short-term mating, the investigators wondered if appetitive aggressiveness might also be an advertisement of good genes. They also speculated that women may be drawn to truculent men during ovulation, when the interest in short-term mating for good genes is at its peak.

In order to test whether fertile women would show a greater preference for warrior types, Biebel and her team conducted an online survey of 1,212 German women. Participants read a fictional scenario of a soldier named “Wilko” who had returned from fighting a war in Afghanistan. From here, the women were instructed to complete three tasks. First, they had to consider Wilko for different types of relationships (i.e., a date, steady boyfriend, life partner, platonic friend, sexual affair, and one-night stand) along a seven-point scale from 0 (not at all) to 6 (most intensive). Second, they rated this fictitious soldier on a seven-point scale along the dimensions of: dominant/submissive; sexually attractive/sexually unattractive; soft/hard; feminine/masculine; rugged/delicate; tough/tender; bad/good; warm/cold; nice/awful; pleasant/unpleasant; friendly/aggressive; unintelligent/intelligent; and healthy/ill. Third, the women reported where they were in their menstrual cycle, in order to assess whether or not they were ovulating (they also identified if they were taking oral contraception).

What did the researchers find? Women preferred aggressive men as short-term mates, and particularly during ovulation. This finding builds on previous work demonstrating that women find male characteristics such as dominance and masculine facial features especially attractive when they are fertile. What's more, this study shows that the male signals of genetic fitness are not just physical, but behavioral as well. At the same time, it is important to underscore that these men were preferred as short-term mates. Dominant men who derive pleasure from being aggressive deliver scant relationship benefits because they pose a threat to the family, show decreased parental investment, and have affairs. Consequently, and as expected, the women in this study preferred less aggressive men for long-term relationships.

What can we learn from this study and related efforts? While it may be bewildering why a woman would fall for the charms of a bellicose man, there's an underlying logic that seems to explain at least part of it: She wants to extract his good genes for posterity. The research also uncovers that the to socially dominant men isn't just psychological—it's undergirded by biology. So while the appeal of an aggressive man may be confusing on an emotional level, an evolutionary lens can bring these tangled motivations into clearer focus.

Vinita Mehta, Ph.D. is a licensed clinical psychologist in Washington, DC, and an expert on relationships, managing and stress, and building health and resilience. Dr. Mehta is also the author of the forthcoming book Paleo Love: How Our Stone Age Bodies Complicate Modern Relationships. Connect with Dr. Mehta on the web at drvinitamehta.com and on Twitter and Pinterest, and find Dr. Mehta's other Psychology Today posts here.