Table 1 Sample analysis paths for different research scenarios concerning the study of sex differences

From: Analysis of sex differences in pre-clinical and clinical data sets

Sample research scenarioSample analysis paths
The process of oogenesis and ovulation in women is qualitatively different from spermatogenesis in men. Women ovulate once each ~28 day cycle, whereas men produce sperm continuously. (Many examples of qualitative sex differences involve reproductive pathways.)Unless comparison of specific aspects of gametogenesis between males and females is the focus, analyze data separately by sex using statistical tests appropriate for the research question.
Pavlovian fear conditioning is conducted in males by quantifying freezing, but when females are tested their ‘deficits’ in a fear response turn out to be an alternative strategy—females also exhibit escape responses [6]. This is a qualitative difference and a quantitative difference.A qualitative sex difference in response to fear conditioning is first demonstrated. Then, individual behaviors (darting, velocity, and freezing) are scored and average values for each behavior for each group are analyzed for each session using two-way repeated measured ANOVA [6].
The link between pubertal timing and depression is qualitatively different in human males and females. In males, there is a linear relation, with late maturing males reporting greater symptoms than early maturing males. In females, there is a quadratic relation, with early and late maturing individuals reporting greater symptoms than those who are on-time. This also suggests a latent difference in the puberty-depression relationship for the sexes.Analyze data separately for males and females. Regression analyses (with polynomial functions) have been used [10]. Other possible approaches include growth curve models and structural equation models [12].
In humans there are sex differences in spatial skills; in fact, the largest cognitive sex difference is in three dimensional mental rotations. This is a quantitative difference, in that—on average—males perform the task more accurately and rapidly than females.Include sex as a variable of interest in planned analyses. Independent samples t tests and factorial analyses can be used; in the latter, sex should be considered as a main and interactive (with other variables of interest) effect.
Female rats are more likely than males to prefer cocaine to a food reward, but cocaine-preferring males and females are similar to each other in terms of behavioral responding and changes in nucleus accumbens dopamine release. This is a population difference.It might be misleading to include sex as a variable of interest in analyses, as frequency (or another nonparametric) analysis points to the nature of the sex difference. But, males and females of similar preference phenotypes may be pooled and statistical tests appropriate for the research question used [9].
Each subtype of red-green “color blindness” is more common in men than women (women require mutations on both X chromosomes versus the single X chromosome). This is a population difference.Phenotypes are similar within each subtype of red-green color blindness, so males and females could be studied together within specific mutation/dichromacy variants, and/or as controls, and sex could be used as a factor in analysis. For inheritance-related research questions, non-parametric analyses of population frequency (e.g., chi-square or logistic regression) or analyses conducted separately by sex are needed.
Pain hypersensitivity following nerve injury exhibits a sex difference in signaling pathways. Males exhibit microglia-mediated signaling, while female signaling is dependent on T cells. Cross-over between these pathways is observed in males lacking testosterone, and in pregnant females or those lacking T cells. This suggests a latent difference.Simultaneous study of males and females and analysis with sex as a variable of interest is necessary to identify these and other differences in pain processing [13]. For the study of sex-specific signaling (e.g., the role of pregnancy in females), data would be analyzed in one sex using statistical tests appropriate for the research questions.
Mice are being tested on a particular behavioral test for the first time—e.g., a same-sex version of the partner preference test developed in voles—and potential sex differences are unknown.Include males and females as study subjects. If qualitative sex differences are present, analyze females and males separately. If differences are quantitative, use sex as a factor in analyses. If not present, use sex as a factor, or pool across sexes. In any of these case, report on the presence/absence and magnitude of sex differences.