Using sex and gender accurately in health research requires a clear understanding of the two concepts because, as Krieger  confirms, "...our science will only be as clear and error-free as our thinking" (p. 656). While much has been written about these concepts, we found that definitions varied, particularly across disciplines. In order to provide clear recommendations of how to use these concepts, we first scanned the literature to assess how and where sex and gender are employed. Our initial scan produced a set of articles which we sorted into the following six categories: gender-based analysis and theories, policy and public health, tools/frameworks, interaction of gender and sex, masculinities/femininities, and examples from research. From here we looked at specific journals and reviewed the reference lists of these papers in order to obtain additional resources. This categorization system permitted us to generally review the issues and advances in conceptualizing sex and gender and helped us to identify gaps in knowledge and obstacles to implementing these concepts in research. Furthermore, our analytic process of assessing the field and reviewing the different usages and definitions of the terms underpinned our own definitions of sex and gender.
In surveying the quantitative literature, we found that gender is often mistakenly used as a substitute for sex; researchers claim 'gender differences' with respect to biology when they are in fact reporting differences according to sex. In the social sciences, where the distinction between sex and gender originated, the concepts are better understood but have evolved theoretically in ways that often seem to defy measurement. The concept of gender in particular has been thoroughly discussed and debated, with many definitions, sub-definitions, and theories offered [20, 21]. However, transferring the latest theoretical developments into functional and operational models for health research and policy has yet to happen, so these important advances remain abstract and often unused in health research. To move forward, standardized definitions need to be accepted by all disciplines and amenable to both qualitative and quantitative research. We developed our definitions in order to incorporate the theoretical advancements in the social sciences in ways that basic scientists could appreciate and use. Our definitions below borrow from the definitions we developed in our primer and are referenced here with permission .
Sex is a multidimensional biological construct that encompasses anatomy, physiology, genes, and hormones, which together affect how we are labelled and treated in the world. Although conceptualizing sex usually relies on the female/male binary, in reality, individuals' sex characteristics exist on a fluid and medically or socially constructed continuum . For example, research has revealed that while the "typical" sex chromosomes are XX for females and XY for males, there are many variations in this genetic chromosomal dichotomy, including XXY, XYY, XXX, and XO (no second chromosome). Therefore our common binary understanding of sex (male/female) is limiting and unrepresentative of the breadth and variety that exist with respect to human sex characteristics. Our common assumption that animals and humans are comprised of two sexes is reinforced by our limited language and has implications for research tools and design .
Sex has an enormous impact on human health in ways not previously understood . For example, research has demonstrated that male and female bodies have innate physiological and hormonal differences that result in different responses to alcohol, drugs, and treatment . In fact, the constitution of the typical female body has inherent differences when compared to the typical male body, from cellular metabolism to blood chemistry. Researchers now claim that "every organ in the body – not just those related to reproduction – has the capability to respond differently on the basis of sex" [, p. 935]. There are important sex-based differences at the cellular level arising from chromosomal dissimilarity. However, while we know that a male liver cell is not the same as a female liver cell, we do not know enough about the exact nature of these differences or whether these differences affect the development of disease or responses to treatment [16, 17]. It is increasingly clear, therefore, that these various cellular differences can potentially create different patterns in the progression of disease in men and women and can lead to differences in health status and outcomes. There is a need to include both female and male animals and women and men in biomedical and clinical research in particular, because results from one group cannot be applied to the other [16–19]. Ignoring the influence of sex in research compromises the validity and generalizability of the findings and can be detrimental not just to the research enterprise but also to the health of individuals .
Gender is a multidimensional social construct that is culturally based and historically specific, and thus constantly changing. Gender refers to the socially prescribed and experienced dimensions of "femaleness" or "maleness" in a society, and is manifested at many levels . The experience of gender is always linked to the social and political context. As such, gender is also intimately connected to social and economic status in systems where maleness is almost universally preferred over femaleness. The valuation of males over females is one way that "gender is a part of all human interactions" and "is a 'stable' form of structured inequality" [, p. 329]. While there is continued debate regarding the dimensions of gender, and its relationships to aspects of diversity, it is widely recognised that gendered experiences and cultural values often result in socially prescribed gender roles that dictate different behaviours, interests, expectations, and divisions of labour for women and men, girls and boys [26–28]. These gender roles are further reinforced by practices, processes and rules that affect gender identity at the individual level, gender relations at the interpersonal or group level, and institutional gender at a macro level .
Gender rolesreflect the behavioural norms applied to males and females in societies that influence their everyday actions, expectations, and experiences. They are expressed and enacted in a range of ways including dress codes, mannerisms, posture, and societal opinions of worthwhile contributions to make as a woman or a man. In some cultures, these roles are sharply defined and differentiated, allowing and disallowing women and men, girls and boys from certain tasks, jobs, opportunities, or spaces [22, 23]. In other cultures, there is more gender equity and the lines between gender roles are more blurred. Either way, gender roles often categorize individuals and control behaviour within institutions such as the family, the labour force, or the educational system .
Gender identitydescribes how an individual sees themselves on the continua of female or male (or as a "third gender" or "two-spirited"), and influences their feelings and behaviours. All individuals develop their gender identity in the face of strong societal messages about the "correct" gender role for their presenting sex, but gender identities are malleable and actively constructed over time and culture, underpinning "an ongoing process of becoming" [, p. 309]. Gender identity is linked to social roles, aspirations, social interactions, behaviours, traits, characteristics, and body image and is influenced by prescribed gender roles and the extent to which individuals accept or resist them. Gender identity is evolving and not always stable. For example, an infant presenting with ambiguous genitalia is often assigned a gender by medical personnel, and then socialized accordingly . Some individuals may experience disjunctions between their apparent sex and their identification with the other gender, leading to transgenderism, and sometimes desires for reassignment (surgical or otherwise). Finally, there are cultural differences that either allow or prohibit expressions of gender identity, such as the "hijra" in India who usually act in feminine ways, but who can be male or intersexed, though they are considered neither male nor female . Growing up in a male or female body affects the gender identity individuals create/develop. For example, growing up female and being raised as members of a less desirable group can make it more difficult for girls to develop positive senses of themselves, which is required for good mental health .
Gender relations refer to how individuals interact with and are treated by others, based on their ascribed gender. Gender relations have a profound effect at all levels of society, and can restrict or open opportunities for individuals . Gender relations interact with "race," ethnicity, class, ability, sexual orientation and other social locations and reflect differential power between women and men and between more or less powerful groups . Gender relations affect personal relationships with others, and also guide interactions within social units, such as the family or the workplace. These relationships have a direct bearing on health . For example, the gendered relationships between men and women have been found to influence the interpersonal dynamics related to tobacco reduction in pregnant and postpartum women . Bottorff et al.'s 2006 study revealed that partner's expectations, support/pressure, and their personal tobacco routines influenced women's attempts to quit smoking during pregnancy and into the postpartum period . Similarly, gendered and racialized relationships between workers and customers affect sales of tobacco to children [31, 32]. DiFranzi et al. found higher incidences of tobacco sales to minors among male clerks  while Landrine et al. found that African-American and Latino children were asked about their age more often than White children when attempting to purchase cigarettes . Furthermore, compliance with smoke free policies in bars has been found to be correlated with bartender gender, where patrons are more likely to comply when served by male staff . The nature and details of these gendered interactions were not always explored; however, these examples illustrate the ways that gender operates relationally and in social contexts.
Institutionalized gender reflects the distribution of power between the genders in the political, educational, religious, media, medical, cultural and social institutions in any society. These powerful institutions shape the social norms that define, reproduce, and often justify different expectations and opportunities for women and men and girls and boys, such as social and family roles, job segregation, job limitations, dress codes, health practices, and differential access to resources such as money, food, or political power. These institutions often impose social controls through the ways that they organize, regulate, and uphold differential values for women and men . These restrictions reinforce each other by relaying social processes of discrimination, inclusion and exclusion, creating cultural practices and traditions that are difficult to change and often come to be taken for granted. There are numerous examples of unequal and differential access for women and girls in particular, to resources that directly affect health and well-being. For example, girls are less likely than boys to be provided with health care, food, or education in many parts of the world . Women are often malnourished due to the priority of feeding other family members first . Even in developed countries, women are less likely than men to have an adequate income, and racialized women even less so, directly affecting their opportunity to achieve good health . Thus, as Lorber and Farrell recognize, "Gender is built into the social order...The major social institutions of control – law, medicine, religion, politics – treat men and women differently" [, p. 1–2].