Men have evolved to prefer dumb, sleepy or drunk women over those with charm and intelligence when they're on the hunt for a one-night-stand, according to a new study.

New findings, published in journal Evolution and Human Behavior, researchers explored the so-called "sexual exploitability hypothesis," an assumption that men are genetically programmed to look for 'easy' and short-term sexual relationships.

The theory is that over time men and women have evolved to developed different reproductive strategies, with women preferring long-term relationships and men preferring more short-term relationships. 

Researchers tested 76 men between the ages of 18 and 47 and found that they rated women who looked vulnerable as being more attractive than those who were more alert.

"Male participants rated photographs of women who displayed varying levels of cues to exploitability," Cari Goetz and her colleagues wrote. The 'cues' included being sleepy, stupid, reckless or intoxicated - and thus easily exploitable.

"We identified 22 cues indicative of sexual exploitability. Nineteen of these cues were correlated significantly with sexual attractiveness," researchers said.

Researchers explained that because ancestral males and females differed in their minimum obligatory parental investment, their willingness to engage in sexual relationships and how much investment to place in a relationship differs between males and females. 

Researchers think that men are naturally born to have little concern for the consequences of casual sexual relationships given that their bodies produce somewhere around 85 million sperm cells per testicle every day. 

In contrast, modern women are born with a brain that is still sensitive to the pregnancy-related consequences of short-term sexual relationships, making them more reluctant to engage in casual sex than men.

Goetz wanted to test out the hypothesis that the average straight man is turned on by characteristics that make a woman look silly or inert, or in other words more "sexually exploitable".

To evolutionary psychologists say the word "exploitable" simply means that a woman is willing and can be more easily pressured into having sex. According to the "sexual exploitability hypothesis" model even if a man has casual sex with a prostitute or a nymphomaniac he is still "exploiting" her. 

Goetz and her team began testing their hypothesis by asking a large group of undergraduate students to put forward some actions, body postures, attitudes and personality traits that might signal vulnerability, such as fatigue, intoxication, or low intelligence.

Participants had submitted a list of 88 signs that suggest that a woman might by especially open to a man’s advances.

Some of the cues on the list were lip lick/bite, over-the-shoulder look, sleepy; intoxicated, tight clothing, fat, short, unintelligent, punk, attention-seeking, immature and touching breast.

Afterwards researchers went on the internet and collected publically available images of women displaying each of these 88 cues and crossed–checked them with a separate group of students who confirmed that the photos indeed matched the cues.

Then researchers showed the pictures to 76 men who were asked to judge the woman's overall attractiveness and how easy it would be to sleep with her using anything from a pickup line to physical force, as well as her appeal to them as either a short-term or a long-term partner.

While images of fat or short women had no effect, meaning that the male participants did not view them as either easy to have casual sex with or appealing as partners, men rated silly or childish-looking women or sleepy or drunk women as being an easy "score".

Furthermore, men rated dumb-looking and tired women were also rated as being more attractive than their more alert or intelligent-looking peers, but only as short-term partners and not as potential girlfriends and wives.

In a follow-up study, researchers wanted to determine whether there was a specific type of man who was most attuned to female 'exploitability' cues by measuring personality traits in the men themselves and found that more promiscuous men who also lacked in personal empathy and warmth were the ones most attuned to female 'exploitability' cues, suggesting that not all men are sleazy when it comes to getting laid.