At Comic-Con, you can be whomever you want to be. 

Ed Quinn/Flickr - flic.kr

The conference is a haven for cosplayers—it's a place where you can attend dressed as your favorite comic book, TV, or film character. For Liev Schreiber and Naomi Watt's youngest son, this meant dressing up as Harley Quinn, a character popularized by actress Margot Robbie's in the movie, "Suicide Squad."

After photos of Schreiber and his son emerged, they went viral on Twitter, as people praised Schreiber for being "an awesome dad."

Of course, there were those who didn't approve of the costume.

Studies have shown it's harmful to restrict children to outdated, binary gender roles.

Dr. Maria do Mar Pereira, of the Centre for the Study of Women and Gender in England, conducted a study on 14-year-olds and gender roles. As she told Think Progress in 2014, "usually we think of gender as natural and biological, but it’s not… We actually construct it in ways that have problematic and largely unacknowledged health risks."

In particular, she observed teenagers in Portugal who knew she was studying them but didn't know her study was focused on gender roles. She found that both the girls and the boys often fell into stereotypical gender roles and behaviors.

kamera kızı/Flickr - flic.kr

For example, as Think Progress noted, "even girls who enjoyed sports often avoided physical activity at school because they assumed it wouldn’t be a feminine thing to do, they worried they might look unattractive while running, or they were mocked by their male peers for not being good enough. The girls also put themselves on diets because they believed desirable women have to be skinny."

The boys, meanwhile, engaged in what Pereira described as "everyday low-level violence" in an effort to appear macho. Her conclusion:

[T]his constant effort to manage one’s everyday life in line with gender norms produces significant anxiety, insecurity, stress and low self-esteem for both boys and girls, and both for 'popular' young people and those who have lower status in school.

Perhaps those who are concerned over a boy dressing as a girl should instead be more concerned about how enforcing strict gender norms can be downright harmful.