Gender labels make attractive people seem unattractive because they make faces harder to process. New research on the "Johnny Depp Effect" may finally explain why women only sometimes prefer feminine male faces.

The "Johnny Depp Effect" is the concept that women tend to prefer men with more feminine faces. In the latest study, researchers from the University of Otago, Warwick Business School, and University of California, San Diego wanted to understand why this phenomenon holds true in only in some situations.

After conducting two separate experiments, researchers found that categorization can seriously affect a person's attractiveness rating. While feminine facial features generally received more approval, researchers found that faces with gender blends were disliked when they were first labeled by gender. Researchers believe that "processing fluency" could explain why gender labels lower facial attractiveness.

Processing fluency refers how difficult it is to perceive, process and categorize something.

Researchers believe that processing fluency can significantly affect attractiveness because more "mental effort can negatively color our initial impressions, even for things that are objectively pretty," study co-author Jamin Halberstadt, a professor at Otago's Department of Psychology, said in a statement.

"The idea we tested is that the mental effort of having to assign a gender to an ambiguous face has a flow-on effect of negatively influencing how we feel about that face," Halberstadt said.

"It has previously been suggested that a woman's preference in male faces vary due to hormonal influences" that sometimes she is subconsciously looking for signs of a "nice dad" who will be a good provider, while other times it is the highly masculine "bad boy" with his 'better' genes," he explained. "However, our research indicates that such changes in preferences can instead be explained by a simple cognitive process."

In the study, researchers concluded that "the variation in attractiveness of gender-ambiguous faces may derive from context-dependent requirements to determine gender membership."

"The more feminine faces are generally preferred, unless the context forces the viewer to put the face into rigid gender boxes," co-author Piotr Winkielman, from the Warwick School of Business and UCSD, said in a statement. "Mental effort can negatively color our initial impressions, even for things that are objectively pretty."

The findings are published in the journal PLoS One.