When Hillary Clinton participated in a televised forum on national security and military issues this month, the chairman of the Republican National Committee, Reince Priebus, tweeted that she was “angry and defensive the entire time — no smile and uncomfortable.” Mrs. Clinton, evidently undaunted by Mr. Priebus’s opinion on when she should and shouldn’t smile, tweeted back, “Actually, that’s just what taking the office of president seriously looks like.”

The implication of Mr. Priebus’s comment was a familiar one: A woman making stern-looking facial movements must be angry or upset. A man who looks the same, on the other hand, is focusing on the important matters at hand.

Today, these stereotypes are playing out on the national stage. Polls indicate that Mrs. Clinton is seen as a more credible candidate than Mr. Trump, yet less trustworthy and likable. This discrepancy is surely rooted, in part, in Mrs. Clinton’s violations of female stereotypes. When she acts “presidential,” she is seen as harsh and cold. In contrast, when Mr. Trump insults his critics or rails against immigrants, many people attribute his anger to the situation (terrorism, the economy) rather than to something about his inner nature. She’s ingenuine; he’s giving voice to the feelings of his fellow Americans.

Most of us recognize gender stereotypes when they’re blatant. But when stereotyping is more subtle, as shown by the facial photo studies, we all succumb and often don’t realize it.

Keep this in mind when you watch the debate on Monday. Will you perceive Mrs. Clinton as running warm and cold, but Mr. Trump as responding to the ups and downs of the debate? When each candidate presses a point, will you see Mrs. Clinton as being irate but Mr. Trump as simply trying to best his opponent? If so, ask yourself: Is it possible that you’re channeling unconscious, cultural biases, and can you rise above them?