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Keywords:

  • biological education;
  • biopsychology;
  • gender bias;
  • gender;
  • homosexuality;
  • sex

Introduction

  1. Top of page
  2. Introduction
  3. The biology of sex
  4. Teaching the biology of sex
  5. Recommendations
  6. References
  7. Supporting Information

Research over the last decades has stimulated a paradigm shift in biology from assuming fixed and dichotomous male and female sexual strategies to an appreciation of significant variation in sex and sexual behaviour both within and between species. This has resulted in the development of a broader biological understanding of sexual strategies, sexuality and variation in sexual behaviour. However, current introductory biological textbooks have not yet incorporated these new research findings. Our analysis of the content of current biology texts suggests that in undergraduate biology curricula variation in sexual behaviour, sexual strategies and sexuality barely feature, even though sex is discussed in a range of contexts. In this aspect, biological teaching is lagging behind current research. Here, we draw attention to new findings in the biology of sex, and suggest how these might be incorporated in undergraduate teaching to provide a more contemporary and inclusive education for biology students.

The biology of sex

  1. Top of page
  2. Introduction
  3. The biology of sex
  4. Teaching the biology of sex
  5. Recommendations
  6. References
  7. Supporting Information

Darwin's strict view of male and female roles most likely reflected the social constraints of his time 1, yet these stereotypic assumptions have influenced evolutionary and biological research for decades 2–4. Darwin characterised males as competitive and eager in the pursuit of females, while females were described as passive, coy and choosy 5. Darwin's theory of sexual selection has been enormously influential and still enjoys tremendous research discourse: in 2010 alone over 1,400 papers were published on the topic of sexual selection (source: Web of Science). While a non-overlapping categorisation of male and female sexes clearly dominates societal attitudes to sex and gender, evolutionary and biological research has shifted away from this static male-female dichotomy in more recent time. For example, we now recognise the great variety and plasticity of sex determination, such as reversible sex change 6, as well as variation in the forms of sexual behaviour (e.g. the role of same-sex behaviour 7). There is greater understanding and acceptance of the variability in behavioural sex differences and even persistent dogmas such as ‘males produce copious amounts of sperm while females produce limited numbers of eggs’ are being whittled away 8, 9.

The combined, albeit slow, paradigm shift in evolutionary biology has culminated in a recent model that proposes the treatment of biological sex (and its associated traits) as a fundamentally plastic reaction norm 4. Gender-neutral models of mate choice have been developed 10, 11 that do not build in sex differences in their basic assumptions of how individuals make reproductive decisions. Ah-King and Nylin 4 argue that this approach allows researchers to acknowledge and deal with variability in biological sex and associated traits in a more objective way.

For humans, significant and consistent variation in sexual behaviour has been recognised since the pioneering work of Kinsey 12, 13, and since 1990 there has been an extremely active investigation of the neurobiological, hormonal and genomic factors that might contribute to this variation 14, 15. As a consequence, issues of sexual orientation and gender-identity have moved beyond the psychological and social sciences to become questions relevant to neuroscientists, endocrinologists and geneticists. In biology, gender is most often used as a synonym for sex, and sex is defined by the size of the gametes, i.e. an individual who produces large sex cells is a female and one who produces small sex cells is a male. In the social sciences, the meaning of gender is different, it means the socially constructed characteristics by which humans identify as females or males, which changes over time and differs between cultures. Thus, gender is performed and not biologically determined. Here, we use the term gender in this latter sense. However, the likelihood and degree of affiliation to a specific gender identity can vary. It is now recognised that in humans sex, gender identity and sexual orientation are distinct traits that can segregate independently, and are each influenced by complex and interacting biological and environmental factors 16, 17.

Teaching the biology of sex

  1. Top of page
  2. Introduction
  3. The biology of sex
  4. Teaching the biology of sex
  5. Recommendations
  6. References
  7. Supporting Information

Judging by the content of current undergraduate biology texts, these new perspectives are not yet incorporated in biology teaching. Introductory-level university biology texts discuss sex exuberantly in diverse contexts that include evolution, behaviour, genetics, physiology, development and botany, but of the nine biology texts we surveyed, none included the new research described above (Table 1). Sex was discussed solely in the context of reproduction (Table 1). Male and female sexual strategies were considered largely invariant by all surveyed biology texts. Variation in sexual strategies or gender identity was discussed by a minority of books (Table 1), and then only in reference to chromosomal abnormalities or congenital adrenal hyperplasia (a clinical hormonal disorder) as examples of disorders that can alter the process of sexual development and thereby induce variation in sexual behaviour. None of the biology books mention any variation in sexual behaviour among animals. Thus, biology textbooks portray a picture of sexual behaviour that is highly stereotypic, contextually limited, and in which variation is both overlooked and portrayed as abnormal.

Table 1. English language introductory-level university text books in Biology, Psychology, and Anthropology published since 1995 (see supplementary data) were surveyed for how they addressed sex.Thumbnail image of
  • Each column represents a different text, filled cells illustrate content included, tones show different disciplines. In this table (and throughout this article), we have followed Bailey and Zuk 7 in our definition of terms. Gender: socially constructed characteristics by which humans identify themselves as males or females. Sexual orientation: a preference for sexual interactions and partnering in humans with individual(s) of the same, opposite, or both sexes. Homosexual: same-sex courtship or copulatory behaviour occurring over either a short-term or involving long-term affiliation. A biological understanding of sex encompasses sex determination, sexual behaviour, procreation, copulation, reproduction, but biological texts do not address sex outside of the broad context of reproduction.

  • It may be reasonable to assume that anthropology subjects cover human sexual diversity but the majority of introductory anthropology texts we surveyed were focused on cultural and social variations and their causes, and did not address sexuality at all (Table 1). Some texts discussed sex-specific behaviour, and how cultural factors can contribute to variation in sex-specific behaviour and gender identity, but only one anthropology text considered how biological factors might influence these traits.

    By contrast, sexual variation is a significant focus in psychology curricula (as reflected by the content of common introductory level psychology texts, Table 1). In this area, sexual strategies are not seen as binary, but as reaction norms in a variable population. The factors that might contribute to variation in sexual orientation and gender identity are discussed by the majority of sampled psychology texts (Table 1). Consequently, we found that currently only psychological text books discuss how biological factors might contribute to variation in sexual behaviour, and then only in the context of human sexual behaviour (Table 1).

    We believe the current coverage of biological sex does not serve biology students, because it is clear that the focus of biology texts is out of step with current research-based perspectives on the topic. Further, any discussion of human sex in biology texts is limited solely to the context of sexual reproduction (Table 1). This perspective presents to a student body the impression that any sexual behaviour that does not fit within this reproductive context and the binary perspective of sexual strategies is outside of scientific discussion and lacks biological validity. Research since 1990 has clearly argued this is not the case 18, and psychology texts have been quick to adopt the new biological information, while biology texts currently perpetuate a heteronormative (depicting heterosexuality as the only natural way of living) and oddly Victorian perspective of human sexual behaviour. The consequence of this is not trivial. We can expect that 10% of an average student body will not conform with exclusive heterosexuality. Current biology teaching ignores this significant natural diversity in human populations, passes up an opportunity to educate on the possible biological bases of this diversity, and perpetuates a limiting and exclusive view of human sex.

    Below, we outline recommendations to develop a biology curriculum that reflects our current understanding of biological sex, including sexual strategies, sexuality and sexual behaviour. We believe incorporating current research-based knowledge into undergraduate teaching delivers the best learning outcomes and an inclusive curriculum for biology students.

    Recommendations

    1. Top of page
    2. Introduction
    3. The biology of sex
    4. Teaching the biology of sex
    5. Recommendations
    6. References
    7. Supporting Information

    There is no reason for biology to limit itself to discussing sex solely in a reproductive and exclusively heterosexual context when much research in behavioural ecology in the last 20 years has emphasised the significance of sexual behaviour beyond a reproductive context and in terms of social affiliation, bond formation and resolution of social conflict in diverse taxa 7.

    In discussing sex-specific traits, current biology texts cite population averages for males and females as if these represent two discrete and non-overlapping populations. A more accurate discussion of research findings would include statements of variation within male and female samples, and the degree to which these distributions overlap. Certainly, for most studies of human sexual dimorphism the overlap is quite large 16, and the meaning of male and female differences can only be understood if presented with an understanding of reaction norms, variance and effect size 19. This approach also makes clear to students an important distinction that can otherwise be misunderstood: that an average simply refers to a summary of a lot of data and not what is ‘normal’. By teaching the biological reality of variation in populations, students benefit from a more sophisticated understanding of quantitative data and the certainty that variation is to be expected.

    Once variation is a theme, it necessitates a discussion of the factors that contribute to that variation. Current textbook coverage of this topic (when it occurs) is also binary – either genetic or environmental influences. Understanding how additive genetic effects can contribute to any complex trait demands an understanding of the meaning of heritability, and this in turn will result in a recognition that genetic and environmental influences interact. A consequence of this approach is the erosion of misconceptions of biological determinism. As researchers in biology, we deal with variation constantly. If we incorporate our understanding of variation into our curricula, the outcome will be a more realistic, current, research focused description of biological sex and sexuality.

    We recognise that research on the biological influences of sexual orientation in humans is a controversial and contested literature (consider for example the ongoing controversy surrounding the reports of a ‘gay gene’ 20) and text books and teachers might be wary of covering material that later may be disproven. However, the facts that variation exists and that such variation is influenced by interacting biological, environmental and cultural factors 14 are not controversial. And where there is controversy we can meaningfully educate by encouraging students to engage with and analyse the controversy. For example, students may be given assignments to encourage exploration of contentious issues, e.g. Do you think the size of the sex cells are what determines behaviour? What information can you find about same-sex sexuality in animals? What evidence is there that human sexual behaviour is influenced by genetics?

    By teaching sex in a broader biological context, we move away from ignoring non-stereotyped behaviour to a more realistic and inclusive understanding. This is important since textbooks receive the authority and prestige of scientific facts. The biological sciences have a strong influence on society's perception of what is ‘natural’, and this impacts on peoples' lives as it affects the interpretation of animal as well as human behaviour, identity formation, political debates and in the long run also legislation. Furthermore, by including in teaching the development of theories and historical as well as current gender biases, we can place the scientific enterprise in relation to societal change and invigorate critical thinking in students. The primary literature provides solid material to support teaching of biological sex in a broader context, and some educational establishments have already recognised the importance of teaching variation in sexual behaviour, for example the Natural History Museum in Oslo, Norway, exhibition ‘Against Nature?’ that features same-sex sexual behaviours among animals.

    We are certain that these revisions – changing from stereotypic portrayals of sexual evolution to a more variable view of sex and sexuality – are necessary in the context of properly informing future generations and contributing to a more realistic, less polarised, starting-point for understanding sex and gender in the future.

    References

    1. Top of page
    2. Introduction
    3. The biology of sex
    4. Teaching the biology of sex
    5. Recommendations
    6. References
    7. Supporting Information
    • 1
      Hrdy SB. 1986. Empathy, polyandry and the myth of the coy female In: Bleier R, ed; Feminist Approaches to Science. New York: Pergamon Press. p. 11946.
    • 2
      Gowaty PA. 1997. Sexual dialectics, sexual selection and variation in reproductice behavior. In: Gowaty PA, ed; Feminism and Evolutionary Biology, Boundaries, Intersections and Frontiers. New York: Chapman & Hall. p. 35184.
    • 3
      Dewsbury DA. 2005. The Darwin-Bateman paradigm in historical context. Integr Comp Biol 45: 8317.
    • 4
      Ah-King M, Nylin S. 2010. Sex in an evolutionary perspective: just another reaction norm. Evol Biol 37: 23446.
    • 5
      Darwin C. 1871. The Decent of Man and Selection in Relation to Sex. London: John Murray.
    • 6
      Munday PL, Buston PM, Warner RR. 2006. Diversity and flexibility of sex-change strategies in animals. Trends Ecol Evol 21: 8995.
    • 7
      Bailey NW, Zuk M. 2009. Same-sex sexual behavior and evolution. Trends Ecol Evol 24: 43946.
    • 8
      Wedell N, Gage MJG, Parker GA. 2002. Sperm competition, male prudence and sperm-limited females. Trends Ecol Evol 17: 31320.
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      Tang-Martinez Z, Ryder TB. 2005. The problem with paradigms: Bateman's worldview as a case study. Integr Comp Biol 45: 82130.
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      Gowaty PA, Hubbell SP. 2005. Chance, time allocation, and the evolution of adaptively flexible sex role behavior. Integr Comp Biol 45: 93144.
    • 11
      Gowaty PA, Hubbell SP. 2009. Reproductive decisions under ecological constraints: It's about time. Proc Natl Acad Sci USA 106: 1001724.
    • 12
      Kinsey AC, Pomeroy WB, Martin CE. 1948. Sexual Behavior in the Human Male. Bloomington, Indiana: Indiana University Press.
    • 13
      Kinsey AC, Pomeroy WB, Martin CE, Gebhard PH. 1953. Sexual Behavior in the Human Female. Philadelphia, PA: Saunders.
    • 14
      Mustanski BS, Chivers ML, Bailey JM. 2002. A critical review of recent biological research on human sexual orientation. Annu Rev Sex Res 13: 89140.
    • 15
      Mustanski BS, DuPree MG, Nievergelt CM, Bocklandt S, et al. 2005. A genomewide scan of male sexual orientation. Hum Genet 116: 2728.
    • 16
      Nelson RJ. 2000. An Introduction to Behavioral Endocrinology. Sunderland, MA: Sinauer Associates.
    • 17
      Poiani A. 2011. Animal Homosexuality: A Biosocial Perspective. Cambridge, UK: Cambridge University Press.
    • 18
      Bagemihl B. 1999. Biological Exuberance: Animal Homosexuality and Natural Diversity. New York: St Martins Press.
    • 19
      Condit CM. 2008. Feminist biologies: revising feminist strategies and biological science. Sex Roles 59: 492503.
    • 20
      Hamer DH, Hu S, Magnuson VL, Hu N, et al. 1993. A linkage between DNA markers on the X chromosome and male sexual orientation. Science 261: 3217.

    Supporting Information

    1. Top of page
    2. Introduction
    3. The biology of sex
    4. Teaching the biology of sex
    5. Recommendations
    6. References
    7. Supporting Information

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