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The Social Neuroscience of Sex; Human Male-Female Similarities and Differences from Individual Biochemistry to Social Behavior

music credit: the disco biscuits - crickets


Slide 1 – Introduction
Three Primary Aims of SNoS Research
1) To search for, and apply a comprehensive theory of human nature, which has yet to be developed for reasons I will explain.
2) To understand how Culture and Biology interact through ontogenetic development, or across an individual’s lifespan – resulting in the uniqueness of each and every one of us.
3) And lastly, to illustrate how the Social Neuroscience can provide answers to both Questions 1 and 2.

Transistion to Slide 2: I would like to begin by defining Sexual Dimorphisms – while also giving a brief disclaimer.

Slide 2 – Sexual Dimorphisms
Sexual dimorphisms refer to differences between males and females of a species in their biological development, cognition and behavior. Which for anyone who has been in a partnered relationship with the opposite sex, their existence should come as no surprise.

However, the most important thing to realize is that any empirically validated or statistically significant difference is at most very minimal. In fact, there are more similarities in all measures both within and between sexes.

So while dimorphisms do exist, they could not provide the foundation for any gender or sex based normative statement regarding ability or potential. All human children, with the proper nourishment and enlightenment, possess ability to mature into a fully functioning, successful and even joyous adult Homo sapien within one of those crazy cultures - we call human life.

The following quote by Melissa Hines, the author of Brain Gender, sums up this disclaimer quite nicely – human beings, rather than simple being male or female, are truly “complex mosaics of male and female characteristics”

Transition to Slide 3: The rest of my presentation will address the primary objectives – beginning with the intellectually history of scientific inquiry and human phenomenology, or theories of human nature, while discussing the emergence of Social Neuroscience.

Slide 3 – Social Neuroscience
In the Western realm, the classically canonized Scientific Revolution of the 16th century, partnered with The Enlightenment and Romantic philosophies of the 17th provided the seeds of contemporary scientific investigation and models of human nature. The growth of world system capitalism and the commoditization of scientific inquiry after WW1 solidified the academic institution into two opposing schools of thought –a split, if you will, between the Natural and the Social Sciences, each of which possess a unique intellectual tradition, various methodologies and particular vernacular for investigating, manipulating and understanding the phenomenon within the natural world.

Through a synthesis of knowledge and a utilization of techniques from each disciplines toolkit – the growing discipline of Social Neuroscience strives to bridge this perceived schism between the natural and social sciences - in order to develop a comprehensive and more accurate model of human nature and sociality. This quote by E.O. Wilson speaks to the goals of social neuroscientific research.

Transition to Slide 4: one of the most important potential contributions of Social Neuroscience will be a withering away of the misguided debates about nature vs. nurture. Throughout my career at Denison, I have come to develop what I call a Dialectical Model of Human Nature.

Slide 4 – Dialectical Model of Human Nature
Along the lines of this model - my senior research combines knowledge and analyzes research from Psychology, Biochemistry, Evolutionary Biology, Sociology, Archeology Anthropology and Philsophy in order to elucidate the origin and existence of human sexual dimorphisms.

T(x) to Slide 5: I am now going to give you a brief overview of the sections within my paper. After which, I would like to share with you, what i believe to be the most interesting and enlightening hypothesis I have come to during my research experience.

Slide 5 – Organization of Research
Evolution and Physiology of CNS
Due to the fact that my research spans multiple levels of analysis, I thought I would be imperative to begin with a discussion on the evolution and physiology of the brain - which is shown in the first row of pictures beginning on the left with a mouse, then chimpanzee and human - with an explicit focus on the cerebral cortex – visible as the purple stain in the coronal slices of row 2.

The take home message of this section is that human brain evolution is reflected by an increase in the complexity, magnitude and plasticity of synaptic connections between neurons within the cortical layers shown in Row 3. Notice in the differences between the cortical layers from amphibians, reptiles across mammals and ending with the human cerebral cortex.

These connections within the cortical layers, especially within layer 1, are absolutely astounding. The external world is sensed and then perception as information from the primary cortical networks diffuse across the matrix of insistently active synaptic connections of the association cortices - reaching out and pulling away from each other in a trail of neuropeptidic communication, reflecting the subjective mirror of experiential memory.

This interaction within the drastically expanded human association cortices reflects the dynamic interaction of culture and biology, as ontgenetic sensations and experiences manipulate the biological regulation that construct and maintains the cytoarchitectural framework of the cortical layers – developing what we would refer to as human nature.

Sexual Reproduction
As a sexually reproducing species, in terms of sexual dimorphisms the most explicit differences between males and females surfaces in behaviors leading up to - and resulting in sexual reproduction.

Traveling along the evolution and social progression of these behaviors from chimpanzees and bonboos to modern Homo sapiens, it becomes clear that sexual selection has been a major force within our Natural History. In addition to this, I review the differences and similarities in the physiology of puberty and the development of sexual orientation, attraction, arousal and climax.

Sexual Differentiation
From this point onward embryonic development begins. As with most mammals, a human embryo is born with bipotent gentialia, having the possibility to become either sex. Although the precise mechanisms have yet to be elucidated, this section reviews current knowledge on the process of Sexual Differentiation - highlighting the genetic, epigenetic and hormonal influences.

Evolution and Social History of Parenting Behaviors
After birth, a child is born eye-wide open into a cultural matrix of social interaction, exchange and communication and it is the duty of a child’s parents to teach, instill or enculturate them into their inherited culture. the social history of parenting system and social dynamics has dramatically influenced our natural history - it is here where i have found one of the most interesting conclusions - to be discussed on the following slide.

In regards to enculturation this section highlights gender stereotypes, gender roles and a number of cross cultural analysizes inorder to hypothesizes which dimorphic traits are remainates of our biological/social evolution, therefore our nature, or remain soley based on culture tradition, one’s nurture.

T(x) to Slide 6: there are a number of thesis i present in this section, but the changes within reproductive dynamics and social organization are what i find to be most interesting.

Slide 6 – Evolution and Social History of Parenting Behaviors
In 1972, R.L. Trivers published a paper entitled “Parental Investment and Sexual selection”. He defined Reproductive Behavior as a function of the time and energy to find, attract, woe and copulate with a partner, or ones' mating effort and the time and energy associated with raising offspring, or ones' parental investment.

As natural history would have it, the females typically invest more effort into parenting because they have a smaller reproductive potential and males invest more effort into mating, or attracting females because they have a higher (meaning more frequent ability) to reproduce.

As a result, males across the animal kingdom have evolved traits to increase their competitive advantage with other males for receptive mates, such as competitive pysches and physiques. Whereas females have evolved the traits such as Synchronized menstruation in addition to concealed ovulation which suggests the traits evolved by human females are mechanisms that allow them to actively select for males who are willing and able to reduce their mating effort and de facto devote more energy to parental investment, perhaps even remaining monogamous.

In his keystone work, The Genetical Theory of Natural Selection, Ronald Fisher proposed that sexual selection doesnt necessarily increase dimorphic traits, but could instead reduce the magnitude of dimorphisms between sexes.

The fossil record of early hominids, from Australopithecus to Homo, indicates that during the past 4 million years there has been a significant reduction in the magnitude of sex differences in physical size, a threefold increase in brain volume, a doubling of the length of the developmental period and a disappearance of related species of Homo.

Geary and Flinn hypothesize, that this coevolving suite of adaptations has resulted from an increased levels of cooperative parenting, or rather an increase in male parental involvement which fosters and enables the developmental acquisition of sociocompetitive abilities - in other words, enables a process of enculturation which I previously mentioned.

While also creating and increasing the complexity of the social structure and organization, providing the child with an intellectually stimulating environment, and allowing children to develop their full intellectual potential...

T(X) to 7: The question is….what trait in males could females observe which told them they would be good fathers?

One of the most prominent cognitive dimorphisms between contemporary humans, across cultures, is that males, on average, perform better on visuospatial abilities where as women, on average, perform better on verbal, communicative abilities.

Slide 7 – The Evolution of Social Cognition and Subjective Consciousness
In regards to human evolution, males are typically viewed as off in small hunting groups tracking and killing large prey for food. this behavior requires relatively little development of communication skills, but a highly develop skills of hand-eye coordination and visuospatial abilities. Whereas, the females remained at homebase forming coalition groups to maintain social cohesion and raise children, this of course, requires highly developed verbal and communicative abilities.

A 1996 study provides evidence that the maternal genome contributes significantly more to the development of brain regions (neocortex and striatum) concerned with not only the processing of sensory information, but with anticipation, forward planning and the execution of these plans. whereas the paternal genome influences more so to regions of the brain important for motivated behavior like (sexual reproduction) found in the hypothalamus and septum. they concluded that our evolved ability to control behavior with thought and intelligence rather than circulating neuroendocrine levels has resulted form changes in social dynamics primarily being differences in selection pressures due to differences in parental lifestyles.

In a 2001 meta-review of studies involving genes and general cognitive ability reported that the incidence of mental disability is 30% > in males than females, and found that the X chromosome contains a significantly higher number of genes that, when mutated, cause mental impairment. 202 x-chromosome linked genes to be exact. Based on the principles of sexual selection have i have already discussed, they suggest that female adaptations to control the reproductive behavior of males eventually lead to a run-away process of evolution favoring the development of exaggerated sexual ornaments in the male, namely improved cognitive abilities.

Lastly, the 2005 study suggests that the X-Chromosome has been engaged in the development of sexually selected characteristics for at least 300 million years and that natural selection has favored the development of x-linked genes that are associated with higher cognitive abilities.

Ultimately, these studies provide a link for the genetic transfer of female sociality to human males throughout evolution as they increased parental investment – This therefore provides evidence for a social or cultural (nurture) trait that has become part of our nature across evolutionary time. Ultimately, this evoluionary history increased the species social complexity and ecological dominance providing Homo sapiens with the adaptations to out compete their contemporary hominids, travel across the earth and develop the global dominance of which we all enjoy.

In addition, it lends to Melissa Hines contemporary observation that we are all truly, “complex mosaics of male and female characteristics”.

thank you for taking the time to review this material, if you have any comments please email me


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