Yale, like other universities, has few women scientists as ladder faculty. Many factors certainly contribute to this gender inequality, but a likely major cause is differential evaluation of women and men in our biased society. Decades of social science research have made this abundantly clear. Unconscious biases lower evaluations of women in male-dominated arenas (like academia) — and of men in female-dominated arenas (like parenthood or nursing).

These biases are not overt: Indeed, most of us like to think we are unbiased. Being objective is a core value in science, so it’s especially hard for a scientist to admit she might not be objective. But research shows we all act in biased ways. It is important to note that the bias against women in science is not due only to men discriminating against women; it is all of us discriminating against women (and minorities). Try taking the online “implicit bias” test of Mahzarin Banaji, formerly a Yale professor, now at Harvard — it offers a real education about one’s own attitudes. In her book “Why So Slow? The Advancement of Women,” Virginia Valian described the origin of this implicit or unconscious bias in “gender schemas” — namely, a set of expectations of women and of men, embedded in our culture and often based on real characteristics, that influence how women and men are judged.

One of the key ways this bias is expressed is in letters of recommendation or personal nominations, which are enormously important in hiring, promotion, invitations to meetings, fellowships, grants, publications (refereeing) and other honors and awards. Yet research shows there are systematic differences in the letters of recommendation submitted on behalf of women and men, which is not yet widely appreciated within academia.

Letters for women are shorter and contain fewer standout words such as “outstanding” or “superstar.” They are more likely to mention women’s personal lives, and in most cases, the mention of gender is explicit. Women are more likely to be compared to other women. Letters for women express more doubt and contain more “grindstone” adjectives (“works hard,” “diligent,” etc.). In our experience, women are asked to write tenure letters for women disproportionately often and their letters are more likely to be discounted or ignored — unless, that is, they are negative, in which case they are given extra weight. That is, women are not reliable if they support other women, but if critical, then they must be more discerning since they would naturally be supporting other women. In other words, women scientists are women first, scientists second. Such a standard is never applied to men; it is never assumed a male scientist supports another male scientist simply because he is male.

The presence of small numbers of women seems to guarantee that bias will kick in. In a study of hiring practices, with artificial and matched resumes, Madeline Heilman found that women can succeed when they are more than 30 percent of the applicant pool but are unlikely to succeed when less than 25 percent. This has obvious ramifications for job searches or tenure letters that include only one token woman on a short list.

As Valian says, expectations of men and women color our judgments even when based on supposedly objective criteria. For example, research shows that men are seen as capable of independent action, oriented to the task at hand and acting on the basis of reason. Women are seen as nurturing and prone to expressing feeling. Men act, women feel. In male-dominated fields such as physics or academic medicine, gender biases lead us to overrate men and underrate women. One paper, published in 2012, showed that physics, chemistry and biology professors rated a resume from a man differently than that from a woman. They were also more willing to mentor or hire the man, and they offered him an average salary about 15 percent higher.

In her book, Valian described how even small, seemingly minor disadvantages can accumulate over a career, leaving women in a decidedly inferior position.

Change happens when leaders recognize unconscious bias and take action. Leaders establish norms. For example, they can make sure colloquia, meetings, prizes and job interviews involve a representative fraction of women. Leaders find and promote talented women. They articulate the importance of gender equity and provide training as needed. They hold people accountable and reward those who do the right thing.

Strong leadership makes a big difference. Information and mentoring are also essential. Some aspects of gender equity are fairly subtle. In many fields, the climate for women is inhospitable. Cultural values unrelated to ability or performance may dominate perceptions of quality (e.g., arrogance, assertiveness, aggressiveness), and indeed may repel some women from the profession. Change has happened in places where strong leaders have set the tone, transforming the institution. Often those leaders are men, since men hold most of the leadership positions.

It is not sufficient, in 2014, simply to follow the status quo. It is not acceptable to say, “Well, we just can’t help it,” there are not very many women or people of color, or they want to have families rather than be professors, or any of the dozens of explanations we have heard. Other institutions have transformed their landscapes. Yale can too.

Shirley McCarthy is a professor radiology. Meg Urry is a professor of physics. Contact them at shirley.mccarthy@yale.edu and meg.urry@yale.edu.