Sex and Gender Differences

• Sex is a biological category, based on a combination of biological traits, such as chromosomes, hormones, gamete size, and genitalia
• Gender is socially constructed with factors relating to an individuals sex and personal identification with their own gender
• Our understanding of sex as a multidimensional biological thing is a social construct and is defined by our current biological epistemology
• The more we learn about human biology, the more our understanding of 'biological sex' changes
• Ancient Greece had 'single-sex' models of biology and the Enlightenment period brought in the 'two-sex' model
• ~0.018-1.7% of people are born intersex - with any sex-related characteristic that does not fit the typical binary notions of male or female bodies
• It is our cultural understanding that places the meaning of sex on bodily features associated with reproduction
• Gender as a concept not truly formalised until the 20th century

  • Freud - concept of gendered behaviour acquired during childhood
  • Money - case study of gender identity following penectomy
  • Butler - performative gender, rejection of fixed identity, gender as something that is created through sociality
• Early psychologists conceptualised gender as a state of 'being', but Butler developed the idea of 'doing gender' - gender is something learnt as a 'performance' that is imposed on us by pervasive heteronormativity
• Transgender people say that 'being' gender has a place in their conceptualisation of gender - Tate et al (2014) argued that 'being' and 'doing' gender should be considered in an integrated way
• Not just 2 genders; increasing identification with non-binary genders in West, indigenous nations in America had more than 2 gender categories but colonisation pathologised and criminalised this. Many cultures have stable, non-binary gender systems: Hijra communities in South Asia, two-spirit people in Native American cultures, Waria in Southeast Asia, Fa'afafine in Pacific Islander communities
• Our understanding of sex and gender being binary and dichotomous was due to our culture and our former understanding - increasing awareness and support that sex and gender isn't fixed or binary
• Study of sex/gender differences in human brain had been largely ignored:

  • Some thought sex/gender was only relevant to functions involved in reproductive behaviours
  • Some thought the differences were not fundamental, or that studying it was distasteful
  • Some people avoid studying it to avoid the potential for neurosexism - misunderstanding of results to fuel prejudice
• Study of sex and gender differences should be continues:

  • Influence can be seen at many levels of analysis
  • Differences may be small, but doesn't mean that they are unimportant
  • Differences and influences can be implications for many things
• Researchers differ in their interpretation of small sex/gender differences: Eliot (2011, 2021) argues that differences are trivial, whereas others argue that even small differences have meaningful consequences
• Hirnstein and Hausmann's (2021) 4 main points in studying small sex/gender differences:

  1. The importance of balanced phrasing
  2. The importance of small effect sizes and meta analysis
  3. Brain-behaviour relationship is unclear
  4. Clinical disorders and the biopsychosocial approach
• Differences in brain structure - macrostructure - brain size:

  • Peters (1991) - male brain larger and heavier on average
  • Ruigrok et al. (2014) - males have larger ICV, TBV, cerebrum, GM, WM, CSF, and cerebellum. Females have larger volumes in right frontal pole, inferior and middle frontal gyri, anterior cingulate gyrus, and lateral occipital cortex
  • Results could be false positives - differences depends on specific analysis method and how body size is controlled
  • Adjusting for differences in total intracranial volume reduced sex differences
• Differences in brain structure - macrostructure - cortical complexity:

  • Luders et al (2004) - spatial frequency of gyrification and fissuration on the cortical surface. Women on average have greater cortical complexity (folding) - more 'compact' brain tissue in smaller brain
• Differences in brain structure - macrostructure - cortical maturation

  • Raznahan et al. (2010) - differences in cortical thickness throughout adolescence.
  • Cortical maturation slower in men in frontal regions (anterior cingulate, PFC, orbitofrontal cortex) - may explain enhanced female performance in PF-mediated cognition
  • Parietal-occipital regions develop faster in men - may explain enhanced male performance in visuospatial tasks
• Differences in brain structure - macrostructure - structural differences throughout brain

  • Richie et al. (2018) - men generally have greater cortical volume and larger volumes in subcortical regions, and women have greater cortical thickness
  • Distributions overlapped a lot - no brain can be identified as 'male' or 'female'
• Differences in brain structure - macrostructure - structural asymmetries

  • Studies suggest there is a sex/gender difference in some lateralised cognitive processes - women tend to be better at verbal tasks and men tend to be better at spatial tasks
  • Planum Temporale - cortical region involved in language comprehension
  • Kulnuch et al. (1994) - men showed asymmetry, women did not
  • Guadelupe et al. (2015) - stronger leftward asymmetry in men
  • Sommer et al. (2008) - no sex/gender difference in asymmetry
  • Plowman et al. (2012) - no sex/gender difference in likelihood of aphasia
• Differences in brain structure - microstructure - size/shape/volume of specific areas

  • Ritchie et al. (2018) - men had greater cortical volume (orbitofrontal cortex, insula, cingulate gyrus) and larger volume (amygdala, hippocampus). Women had greater cortical thickness overall, but particularly in the parietal lobe
• Differences in brain structure - microstructure - GM:WM ratio

  • Absolute white matter volume generally higher in men, and absolute grey matter volume higher in women
  • Allen et al. (2003) - women have a higher GM:WM ratio - effect of sex/gender greater on white matter - women have less white matter and more grey matter
  • Joel et al. (2015) - focused on internal consistency (how much brains align with 'typical') - largest sex/gender differences in grey matter volume was in left hippocampus and left caudate and low internal consistency
• Differences in brain structure - microstructure - synaptic and neuronal density

  • Stark et al. (2007) - higher number of neurons in frontal and temporal cortex in men
  • Witelson et al. (1995) - neuronal density 11% higher in women in the temporal lobe
  • Alonso-Nanclares et al. (2007) - women showed lower synaptic density in all layers of the temporal neocortex
• Differences in brain structure - structural connectivity - macrostructural

  • Posterior subsections of the corpus callosum are larger in women, some inconsistent finding. Ardekani et al. (2013) controlled for differences in overall brain size and found that the corpus callosum was larger in women
  • Ingalhalikar et al. (2014) - men exhibited greater intrahemispheric and women exhibited greater interhemispheric structural connectivity - but this is driven by brain size; smaller brains more likely to have greater interhemispheric connectivity, and are more likely to be female
• Structural connectivity:

  • Studies report measures of fractional anisotropy and mean diffusivity as indicators of white matter organisation and integrity
  • High anisotropy = good white matter integrity
  • High mean diffusivity = poor white matter integrity
• Differences in brain structure - structural connectivity - microstructural

  • Westerhausen (2003, 2011) - higher fractional anisotropy and lower mean diffusivity in the corpus callosum in men
  • Some evidence shows higher FA in corpus callosum in women and other evidence suggests there is no sex/gender differences in global FA and MD
  • Other evidence suggests sex/gender differences in white matter microstructure become non-significant after controlling for differences in intracranial volume
• Differences in brain structure - the human brain 'mosaic'

  • Joel et al. (2015) - there is considerable overlap between men's and women's brains throughout samples, and most individual brains are characterised by aspects of an 'average' or 'typical' 'male' or 'female brain'
  • Brains should be conceptualised as 'mosaics of features' - small and reliable sex/gender differences at population level, but not in the individual
• Differences in brain function - Functional Cerebral Asymmtetries

  • These are specific cognitive processes that are lateralised to a specific hemisphere
  • Studies suggest that FCAs can vary according to sex
• Differences in brain function - FCAs - lesion patients

  • Verbal IQ - men had intact verbal IQ when they had a right hemispheric lesion, but women had damaged verbal IQ when they had a lesion to either hemisphere - men have an asymmetry for language to the left
  • Spatial impairments - men with right hemispheric lesions demonstrated poorer performance, performance for women was intact no matter the side of the lesion
  • Women are more bilateral (smaller FCAs) and men are more lateralised (bigger FCAs)
• Differences in brain function - FCAs - behavioural studies

  • Right ear advantage - women demonstrate smaller REA, high masculine men had higher REA
  • Visual tasks - larger right hemispheric responses in men for facial processing task
  • Meta-analysis conclude small but reliable sex differences in FCAs exist at population level - men have larger FCAs than women on average
• Differences in brain function - FCAs - sex hormones

  • Hodgetts et al. (2015) - high levels of estradiol (follicular phase) reduces language lateralisation - sex differences found may be due to cycle phase of female participants
• Differences in brain function - task-related brain activity

  • Mental rotation - men usually perform better. Women exhibit more bilateral activity in parietal and premotor areas whereas men exhibit more lateralised activity in parietal and visual areas, even when there are no performance differences
  • Cognitive control - working memory tasks under negative emotion induction was associated with activity in prefrontal and parietal areas for men and the amygdala and orbitofrontal cortices for women
• Differences in brain function - task-related functional connectivity

  • Mental rotation - women showed greater activity in the dorsomedial PFC (top-down processing) whereas men showed greater activity in the basal ganglia precuneus, sensory cortices and deactivation of the parieto-insular vestibular cortex (bottom-up, automatic processing)
  • Emotion and cognition - only women showed a suppression of activity in the anterior cingulate cortex (top-down)
• Differences in brain function - resting-state functional connectivity

  • Default Mode Network - comprised of the dorsal and ventral medial PFC, posterior cingulate cortex, precuneus, and lateral parietal cortex
  • rs-fMRI studies show stronger functional connectivity in the DMN of women
  • Human Brain 'mosaic' - AI model could classifiy 65-75% of brains from rs-fMRI data, but mostly based on most different and distinct areas - brains are not sexually dimorphic, but specific regions can be more 'male' or 'female typical'
• Differences in brain function - resting-state functional connectivity

  • Sex hormones - women showed higher functional connectivity in 2 resting state networks, particularly frontal regions
  • DMN functional connectivity stable in men but fluctuated in women - increased connectivity in left PFC during menstrual phase
  • Auditory network - higher connectivity in men but not effect of cycle phase in women
  • Sex/gender differences in resting state connectivity may depend on cycle phase. Frontal areas more sensitive to hormone fluctuations
• Differences in cognition - visuospatial abilities

  • Medium to large effect sizes favouring men for mental roation
  • Collins and Kimura (1997) - more difficult tasks more likely to demonstrate larger differences
  • Men slightly better in other visuospatial tasks with smaller effect sizes - spatial perception and visualisation, shown from young age (Geringer and Hyde, 1976)
  • Women outperform men in spatial location memory tasks (Voyer et al., 2007) - other research contradicts
• Differences in cognition - verbal abilities

  • Women outperform men - verbal tasks, verbal fluency, reading and writing skills, phonemic fluency
  • Magnitude of difference dependent on task used and variables controlled for
  • Mathuranath et al. (2003) - examined effect of age, education, and gender, finding no significant sex/gender differences. Education influenced letter fluency and older age inversely affected category fluency
• Differences in cognition - attention

  • Women outperform men
  • Attention orientation - women more susceptible to distraction from invalid cues (Merritt et al., 2007) but better able to make use of cues, compared to men who ignored all (Bayliss et al., 2005)
  • Sustained and divided attention - men better at both (Pletzer et al., 2017) - not consistently replicated (Kheloui et al., 2023)
• Differences in cognition - memory

  • Women outperform men on tests of episodic memory and object location memory (Voyer et al., 2007)
  • Men show greater variance in verbal and spatial episodic memory. Women show greater variance in route memory (Asperholm et al., 2020)
  • Women outperform men in recall and recognition for verbal episodic memory (Hirnstein et al., 2023)
  • Memory can be influenced by sex hormones, particularly for memory processes that involve cognitive control