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Stop wondering if you've been friend-zoned or not. According to a new scientific study, there's a precise way to tell whether or not a lady has mentally filed you away. Ask her to take a walk.

The pace at which we walk may have to do with how we feel about the person we are walking with. According to a new study published by the online scientific journal PLOS ONE, men and women adjust their walking speed to save energy for reproduction. So when you’re trying to pass that couple holding hands and walking at a snail's pace at the mall, there might be a biological reason for it.

Other animals are known to do this, according to the study's write-up. They “adopt effective and efficient mobility strategies” to save time and energy for other activities and to protect the fertility within their population. Fertility rates go up or down depending on daily walking distances, and the same may apply to humans.

When humans walk, there are two factors that come into play: 1) cost of transport, and 2) speed that will determine how much energy we use. Cost of transport is the energy consumed to travel a certain distance.

There is a certain speed, neither too fast nor too slow, that is just right and doesn’t waste too much energy. This is called optimal speed. Any deviation from optimal speed results in more strain on the body and caloric output.

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Since humans come in all shapes and sizes, we all have different optimal speeds. This is where it gets interesting. Generally, males are faster than females. When walking in groups, someone must “pay the energetic penalty of deviating from his or her optimal speed.”

And who pays this price? Men pay the price! And we do it because we want to preserve your reproductive system ladies. Aren’t we the best?

The male reproductive system’s is not significantly affected by high energetic outputs. For females, lack of energy affects ovarian function, “eliminating the possibility of conceiving until energy balance is restored.”

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For guys walking with other guys, their pace was faster than optimal. When females walked together, their pace was slower.

The results indicate that males will bear the energetic penalty to walk at the female’s pace when romantically involved in order to protect the fertility of females.

So the ultimate question is: when dudes walk slower, is it because they want children or because they want to get lucky?