The male brain has a tough time during development. If we look at all disorders of the nervous system, from autism to stuttering, the risk is higher in males – sometimes markedly so. Even in injuries, males fare worse than females.
But after puberty, the risk of disease is higher for female brains. We see a higher incidence of autoimmune disorders and schizophrenia in older females, for example.
There may be different factors at play, but there must be a biological aspect to this too. It’s essential that we understand how the brain develops in the male and female body. Is the difference in disease risk during development because of male vulnerability, female resilience – or both?
A new policy requires researchers to “account for sex as a biological variable” when they apply for funding from the National Institutes of Health (NIH) in the US. This means they need to state the sex of any animals or people they are studying, including for basic biological research, not just clinical trials. The aim is to improve our understanding of sex differences and help improve everyone’s health.
To do this, we need new tools – not only those that help us understand sex differences and peer into the inner workings of the brain, but also tools that help us organize and share data so researchers can ensure they have the necessary data for their work. Tools like Hivebench, an electronic lab notebook, help researchers manage their protocols, experiments and results, making important data more widely usable. Other tools, such as Mendeley Data, assist with the searching and sharing of that data.
Those results can then be published and accessed. Across Elsevier’s neuroscience portfolio, researchers are publishing work on sex differences, highlighting the importance of considering sex in experiments and helping researchers understand the issues and adjust to the new NIH guidelines. In this collection, we have gathered the latest research across a range of topics, and two forthcoming special issues on sex differences in the Frontiers in Neuroendocrinology and Hormones and Behavior journals will explore the issue further.
Assuming sex differences
As scientists, we want to reduce variability in our experiments and control as much as possible so we can see the mechanisms behind diseases. Researchers used to think that females were too variable because of hormone fluctuations and other things, so until relatively recently the vast majority of biological and medical research was done on males. We assumed males and females were the same until proven different.
But this was a false assumption that has led to significant holes in our knowledge about the female nervous system, including how it responds to several different drugs. Basic research should be applicable to everyone and just as accurate and relevant for women as it is for men.
In our experiments, we should therefore assume males and females are different until proven the same. This means when we work with animals and humans, we need to work with an equal number of males and females, or at least state the sex of the individuals. That’s an approach the new NIH policy intends to enforce; in order to get funding for biological research, we now have to consider sex in our studies.
In one of the articles in this collection, researchers at the University of Pennsylvania Perelman School of Medicine in the US studied the MRI scans of 875 young people to see whether differences in anxiety and depression symptoms in men and women are connected to blood flow in the developing brain. Their findings, published in Biological Psychiatry, show that changes in the way the blood flows to the brain during adolescence differ in men and women, and that this could be a critical element of the differences in anxiety and mood in men and women.
Exploring extreme biases
Other disorders show even bigger sex differences; the incidence of autism spectrum disorder is heavily biased towards males, for example. My own work focuses on brain development and the role of inflammatory signalling molecules. The male brain has more of these molecules, and there is emerging evidence that they are involved in determining brain maleness. It also appears that people with autism spectrum disorder have overactive neuroimmune systems, which is connected to these signalling molecules.
In a recent paper in Biological Psychiatry, we reviewed this evidence and proposed that the process of brain masculinization, which occurs naturally, puts males at higher risk of autism spectrum disorder. We believe this is because the process puts the brain in a vulnerable position, and inflammation at critical points could result in developing the disorder.
Despite the extreme bias we see in many disorders, and the differences in the ways the male and female nervous systems respond to different drugs, there are still criticisms of researching sex differences. Some people legitimately worry about playing to stereotypes, for example, by saying that behavior is preordained by gender – like comparing skills in men and women. Is the way a rat runs a maze indicative of ability in boys and girls?
Calling this kind of research “neurosexism” is not helping. Some people claim that studying the biological basis of disorders and behaviors is fueling sexism and hampering equality. Yet opposing this research means ignoring the evidence that there are biological differences. This could jeopardize equality in the quality of research and treatment for men and women. Only by understanding sex differences can we make sure men and women receive the best possible medical treatments.
Improving our research
In the 1990s, the NIH required clinical trials to include women, children and minorities, unless there was a good reason not to. But with no such requirements for preclinical research, scientists were often surprised to find adverse effects in women only. This stopped eight out of 10 trials during a 3-year period from 1997 to 2000, wasting millions of dollars and needlessly jeopardizing lives.
Now we have the opportunity to identify sex differences at a much earlier stage. The NIH policy is not mandating researchers to study sex differences directly, but to include sex as a variable – that’s something different. In practice, this means scientists have to make sure some samples come from males and some from females, and they have to break down the data by sex to see if there’s something to look at more closely.
There are, however, obstacles to studying sex differences, which researchers at Tulane University and the University of California outlined in their “Guide for the Design of Pre-clinical Studies on Sex Differences in Metabolism,” published in the Cell Press journal Cell Metabolism. The authors wrote:
In virtually any physiological study, the focus on a single sex threatens to limit the impact of research findings, as results may be relevant to only half of the population.
To overcome this threat, they say we urgently need novel research tools. The tools we work with today are not suitable for the more complex and sophisticated research we want to do. But the more significant problem they identify is the need to improve our familiarity with sex differences research – not just as investigators but also reviewers.
For example, we need tools to give us a better understanding of how the sex chromosomes impact the autosomes. We need to understand imprinting better, as it deferentially effects males and females. We need tools for understanding hormones better, and we need the ability to see the steroids in the brain.
We’re well on our way: scientists are already making changes in the way they design experiments, and there is a growing body of research on sex differences in neuroscience. At the Society for Behavioral Neuroendocrinology’s Annual Meeting in June 2017, many of the talks were about sex differences in experiments, from questioning the “hardwiring” of the brain to looking at how the male and female brains are regulated differently at the cellular level.
At the Organization for the Study of Sex Differences (OSSD), we want to enhance this by facilitating interdisciplinary communication and collaboration. Working with other scientists and clinicians from diverse backgrounds can help us understand sex differences better and build on the research we already have to improve the health of men and women.
One of the big questions going forward will be how are we ever going to measure the impact of the new policy? One way will be looking in journals and seeing if all studies are reporting the effects of sex and disaggregating data by sex. We’ll also see evidence of a shift in the next round of grant applications. But the real results will start to show in the grant renewals: if a researcher proposed to study male and female samples, will we look back five years down the line and see that it happened?
You can read some of the latest studies on sex differences in this collection, which is free to access for the next three months.
- “Sex differences in brain and behavior in adolescence: Findings from the Philadelphia Neurodevelopmental Cohort,” Neuroscience & Biobehavioral Reviews
- “Predator odor exposure of rat pups has opposite effects on play by juvenile males and females,” Pharmacology Biochemistry and Behavior
- “Elevated Amygdala Perfusion Mediates Developmental Sex Differences in Trait Anxiety,” Biological Psychiatry
- “Convergence of Sex Differences and the Neuroimmune System in Autism Spectrum Disorder,” Biological Psychiatry
- “Study of sex differences in the association between hip fracture risk and body parameters by DXA-based biomechanical modeling,” Bone
- “Sex differences in cardiovascular disease – Impact on care and outcomes,” Frontiers in Neuroendocrinology
- “Sex differences in microglial colonization and vulnerabilities to endocrine disruption in the social brain,” General and Comparative Endocrinology
- “Sex differences and sex hormones in anxiety-like behavior of aging rats,” Hormones and Behavior
- “Sex-related differences in the skeletal phenotype of aged vitamin D receptor global knockout mice,” Journal of Steroid Biochemistry and Molecular Biology
- “Role of neuroinflammation and sex hormones in war-related PTSD,” Molecular and Cellular Endocrinology
- “Predisposing factors for chronic angle-closure glaucoma,” Progress in Retinal and Eye Research
- “Sex differences in renal transcriptome and programmed hypertension in offspring exposed to prenatal dexamethasone,” Steroids
- “Sex differences in autonomic response and situational appraisal of a competitive situation in young adults,” Biological Psychology
- “Sex differences in verbal working memory performance emerge at very high loads of common neuroimaging tasks,” Brain and Cognition
- “Gender Differences in Episodic Encoding of Autobiographical Memory,” Journal of Applied Research in Memory and Cognition
- “Age and gender differences in implicit motives,” Journal of Research In Personality
- “Perceived emotional intelligence as a predictor of depressive symptoms during mid-adolescence: A two-year longitudinal study on gender differences,” Personality and Individual Differences
- “Sex steroids and the GH axis: Implications for the management of hypopituitarism,” Best Practice & Research: Clinical Endocrinology & Metabolism
- “Differences between genders in relation to factors associated with risk of diabetic foot in elderly persons: A cross-sectional trial,” Journal of Clinical & Translational Endocrinology – open access
- “A Guide for the Design of Pre-clinical Studies on Sex Differences in Metabolism,” Cell Metabolism – Cell Press