It's a Hen, not a Him: sex bias in neuroscience research

Tales from Lab Meeting

I’d like to conduct a survey.

Q1. How many times per lab meeting can I interrupt a discussion if someone refers to a female chicken as “he”?

This survey isn’t (solely) hyperbolic. I could actually use some advice over here. The comment section is open.

Do I interrupt my PI when he does it? The post-docs? Should I comment the first time it happens in each lab meeting, or the first time someone who isn’t the head of my lab does it.

As folks out there may well know, I work in a lab that studies behavior in chickens (amongst other things). After an infamous incident involving a newly sexually mature rooster, a surprisingly thin-walled animal facility, and the Shenoy lab monkeys*, my lab exclusively keeps female adult chickens.

And yet, despite knowing with absolute certainty that our birds are females, the vast majority of discussions during lab meeting refer to the hens using masculine pronouns:

“And then he pecks on the zeroing cross.”

“Does he prefer pecking at higher or lower targets?”

“Have you trained him on the cue-based task yet?”

A Literary Interlude

A seminal component of my childhood reading experience was the works of Tamora Pierce. Her books often feature a young female protagonist, taking on non-traditional gender roles in a medieval fantasy setting. One scene, in which the main character is the only female in a classroom full of males, seems particularly apropos.

Kel opened the book and pointed to the author’s name. “Sir, the writer is my father.”

The master of ceremonies snatched the book away and scowled at the title page. “What of that?” he demanded. “The child does not have all of his father’s knowledge.”

All of her father’s knowledge, thought Kel irritably.

“Excuse me, Master Oakbridge,” Neal said in his friendliest voice, “but Kel doesn’t have all of her father’s knowledge. Not his.”

Dropping her blank Yamani mask-face, Kel glared at him.

Oakbridge also glared at Neal. “The majority of you are lads. Proper usage calls for male pronouns when males are part of the group.”

“Except that you addressed Kel alone, which then demands the exact term.”

-- Tamora Piece, First Test

Bias in Science

At least on the second floor of the Fairchild Building, it is a truth universally acknowledged, that the pervasive STEM field gender gap, and the inherent biases against women in science that everyone, even female scientists, hold, are a problem. This blog post isn’t about these issues - I’ll merely comment that for those of you who haven’t yet read it, Stanford Professor Jennifer Raymond’s recent Nature article, Sexist Attitudes: Most of us are biased, is excellent.

Last week, at the Stanford Neurosciences PhD program retreat, held in a very swanky Pajaro Dunes Resort, Dr. Annaliese Beery (Smith College), gave an absolutely fantastic seminar.** By audience request, after she discussed her primary research on the neural basis of social bonds between rodent peers, Dr. Beery talked about a recent review she conducted with her post-doc advisor, Dr. Irving Zucker (UC Berkeley), on sexual biases in research on mammals. Their review was published in 2011, in the journal Neuroscience & Biobehavioral Reviews (Beery and Zucker, 2011).

Their basic claim? Female mammals are underrepresented in animal models, and for no good reason.

For 10 areas of biological research, e.g. neuroscience, Drs. Beery and Zucker conducted a literature review. The took four journals with high impact factors dedicated to publishing peer-reviewed articles on the chosen subject. In the case of neuroscience, the Journal of Neuroscience, Neuroscience, Journal of Comparative Neurology and Nature Neuroscience. For each journal, they surveyed the first 20 primary research articles published in 2009 that involved research on mammals. They determined whether the authors of those 20 articles listed research subjects as males, females, both or unspecified. Furthermore, if a study used both males and females, Drs. Beery and Zucker determined whether data were analyzed by sex.

(Some of) The Results

Fig. 1. Distribution of studies by sex and field in 2009. (A) Percent of articles describing non-human animal research that used male subjects, female subjects, both male and female subjects, or did not specify the sex of the subjects. Modified from Beery and Zucker, 2011

Across all 10 fields surveyed, 8 had biases towards using male animal subjects. These fields were: neuroscience, pharmacology, physiology, endocrinology, zoology, behavioral physiology and behavior. A skew towards using female subjects was found in the last 2 fields, immunology and reproduction.

Neuroscientists, meet me at camera three

Hi there, folks. Let’s ignore the other fields included in Annaliese’s study, and focus on the one that matters most to us (that’d be neuroscience). As of 2009, articles published in the aforementioned journals are rocking an impressive 5.5:1 bias towards reporting on only males versus only females. That’s the largest bias of all 10 biological fields.

But you know what’s worse that being called the most sexually biased field? Well, two things. 1) The fact that 22-42% of articles in neuroscience, physiology and interdisciplinary biology journals failed to specify the sex of the animal in question. 2) Of the ~10% of neuroscience articles that did report using both males and females, only 20% of those studies analyzed their data by sex.

Why a sex bias?

Apparently, the tradition of excluding females from research subjects is based on the assumption that for many animal behaviors, estrous or menstrual cycles result in higher variability amongst female subjects than seen in male subjects. This assumption has been directly challenged by several studies, but the common wisdom remains. Indeed, during my days as a rodent physiologist, I was trained that males were preferable for experiments because they would exhibit less response variability.

What should be done?

Drs. Beery and Zucker make a strong case that exclusion of females in non-human animal research is an issue whose correction will improve the quality of scientific research. I cannot think that many people will disagree with the authors that, "The genetic sex and hormone cycles of female animal models profoundly affect biological processes that remain urgently in need of investigation if we are to fulfill the mission of improving quality of life for women as well as men."

Beery and Zucker propose co-opting the NIH guidelines that have already successfully increased inclusion of females in clinical studies. Specifically, they suggest applying those guidelines to non-human animal research as follows:

1. If male and female animal models are thought to differ in response to an intervention then the study must be designed with adequate sample sizes to answer the question for each sex. 2. If prior research strongly indicates that there are no significant differences between male and female animals, then sex is not required in subject sex selection, but study of both males and females is both feasible and encouraged. 3. If information about the existence of sex differences is absent or equivocal then both sexes should be studied in numbers sufficient to permit valid analysis. 4. Study of mechanisms underlying sex differences should be a high priority. 5. Outreach training activities offering practical suggestions and additional sources of information should be made available by the NIH to help investigators design studies that fully incorporate female animals. 6. The review process for extramural funding should treat inclusion of female animals as a matter of scientific merit that affects funding.

All right, it’s time for a senior grad student moment.

Yes, carefully designed studies that seek to discover new sexually dimorphic traits should be done; such an effort can only advance biomedical research.

But, I am not naïve; I am under no illusions that the majority of neuroscientists will have either the inclination or the resources to expand their studies to include a full-fledged investigation of sexual-dimorphic features. I certainly cannot imagine attempting to include such a study in the last R01 my lab submitted to the NIH; neither do I think our reviewers would have been pleased with such an unfocused, likely unnecessary, series of proposed experiments. Yes, the proposed comparisons of responses in different sexes is ideal in theory, but in practice, hard to motivate.


I think that there are at least a couple practical, implementable steps we (neuroscientists, graduate students, those in the trenches) can take.

 What should we demand of ourselves?
  1. Think about your experimental design, and make a deliberate decision as to whether you will use only one gender or both. Be prepared to defend that decision.
  2. When you do an experiment, include gender in your experimental notes. When you publish, report the gender of your research animals. The Nature publishing group has recently removed length restrictions on its methods sections. Where they lead, hopefully other journals will follow. Folks publishing in a NPG journal, you now have no limitations to hobble the detailed reporting of your experimental methodology. Use your power wisely.
  3. If you have the data, either do the comparison between effect sizes in males and females, or mention your decision to not do the comparison. (Assuming you have sufficient sample sizes to make such a comparison statistically meaningful. More on that subject in an upcoming post by Kelly Zalocusky.) Let's avoid relegating this issue of sex bias to "out of sight, out of mind" status.
What’s next for me?

As I said, as convincing as I found Beery and Zucker’s discussion of the importance of research into sex differences, I will continue to study the neural mechanisms underlying attentional modulation of sensory information processing. On the less drastic scale, given the age and breed of chicken I employ, it is not feasible for me to identify the gender of my animal subjects. Birds are notoriously difficult to sex before they develop secondary sexual characteristics. But even though I will not be able to track the gender of each experimental subject, I can explicitly state in my methods sections that I did not select for any particular gender; that I assumed a 1:1 ratio of male and female chicks.

And as for the behavior hens, well, while writing this article, I checked the most recent draft of a soon-to-be-submitted R01, to see whether we had specified the gender of the bird. We hadn’t. Guess I know what the next edit I make will be. Maybe I’ll mention those male pronouns to my PI at the same time.




Beery and Zucker (2011). Sex bias in neuroscience and biomedical research. Neuroscience and biobehavioral research. Volume 35, Issue 3, January 2011, Pages 565–572

* Turns out, monkeys who can’t sleep at night because a rooster is constantly crowing, will fall asleep during the day. Which gets in the way of experiments.

** I cannot sing Dr. Beery’s praises enough.  If there are any readers searching for researchers to invite to their seminar series, I give a full-throated endorsement of inviting Dr. Beery. You won’t regret it. Again, here’s her lab website. I found her research on the role of neuropetides in regulating social behaviors to be particularly interesting. Not to mention adorable (see right). And her field research stories are great (permits for importing/exporting lab materials from South American countries can be … hilariously tricky).

And one last thing: Some non-sex-related findings findings that surprised me not at all:

Fig. 3. Species use in animal studies by subject area in 2009. Six fields (general biology, immunology, neuroscience, physiology, pharmacology, and endocrinology) relied on rodents in 80% or more of animal studies. (from Beery and Zucker, 2011)

Have you ever gotten the feeling that everyone and their post-doc advisor is using rodent models in neuroscience? Turns out “everyone” equals approximately 85% of published articles, at least as of 2009. The authors note that a separate analysis of historical trends in animal research shows that in the first 2 decades of the 20th century, fewer than 10% of animal studies used rodents. Rates of rodent use started to skyrocket between 1969 and 2009; largely because of the growing popularity and genetic tractability of mice.


Astra Bryant

Astra Bryant is a graduate of the Stanford Neuroscience PhD program in the labs of Drs. Eric Knudsen and John Huguenard. She used in vitro slice electrophysiology to study the cellular and synaptic mechanisms linking cholinergic signaling and gamma oscillations – two processes critical for the control of gaze and attention, which are disrupted in many psychiatric disorders. She is a senior editor and the webmaster of the NeuWrite West Neuroblog