Page 14 of Dr. Louann Brizendine's recent work of popular neuroscience, The Female Brain, offers a vision of the "innate biological destiny" of men and women in which key cognitive differences are determined long before birth:
Until eight weeks old, every fetal brain looks female -- female is nature's default gender setting. If you were to watch a female and a male brain developing via time-lapse photography, you would see their circuit diagrams being laid down according to the blueprint drafted by both genes and sex hormones. A huge testosterone surge beginning in the eighth week will turn this unisex brain male by killing off some cells in the communication centers and growing more cells in the sex and aggression centers. If the testosterone surge doesn't happen, the female brain continues to grow unperturbed. The fetal girl's brain cells sprout more connections in the communications centers and areas that process emotion. How does this fetal fork in the road affect us? For one thing, because of her larger communication center, this girl will grow up to be more talkative than her brother. Men use about seven thousand words per day. Women use about twenty thousand. For another, it defines our innate biological destiny, coloring the lens through which each of us views and engages the world. [emphasis added]
"A huge testosterone surge" that "[kills]... communication" and "[grows] ... sex and aggression". Already in the eighth week of fetal development. This is strong stuff, reminiscent of some of the fiercest anti-male rhetoric of thinkers like Mary Daly, who believes that the world would be better off without any men in it. And unlike Daly, Brizendine supports her vision with a dense series of end-notes referencing big-time scientific journals like Science, Endocrinology, and Nature Reviews Neuroscience.
The trouble is, this vision is mostly not true. As far as I can tell, it's Brizendine's personal echo of current sexual stereotypes, unsupported by any of the scientific papers that she cites.
The end-notes for page 14 of The Female Brain are on page 192. Let's take Brizendine's references one line at a time, and see what support she offers for her powerful vision of sexual stereotypes determined by fetal brain development.
Until eight weeks old, every fetal brain looks female -- female is nature's default gender setting. If you were to watch a female and a male brain developing via time-lapse photography, you would see their circuit diagrams being laid down according to the blueprint drafted by both genes and sex hormones.
14: ". . . both genes and sex hormones.": Glickman 2005; Arnold 2004.
The two end-note citations resolve to four papers in B's bibliography:
1. Glickman, S.E., R.V. Short, et al. (2005) "Sexual differention in three unconventional mammals: Spotted hyenas, elephants and tammar wallabies." Hormones and Behavior 48(4):403-17.
2a. Arnold, A.P. (2004) "Sex chromosomes and brain gender." Nat Rev Neurosci 5(9):701-8.
2b. Arnold, A.P., and P.S. Burgoyne (2004). "Are XX and XY brain cells intrinsically different?" Trends Endocrinol Metab 15(1):6-11.
2c; Arnold, A.P., J. Xu, et al. (2004) "Minireview: Sex chromosomes and brain sexual differentiation." Endocrinology 145(3): 1057-62.
What these recent papers add to the earlier consensus is an emphasis on the role of differential gene expression in the brain, in addition to the previously-established effects of testosterone and antimüllerian hormone. Arnold (2004c) in particular suggests that the female development is not entirely "passive", in the sense that X-inactivation ("the process of transcriptional silencing of one of the two X chromosomes in each non-germline XX cell") might be incomplete, so that "[s]ome X genes ... escape inactivation, and therefore could be expressed at a higher dose in females".
None of the papers cited, however, offers any information at all about the nature or magnitude of sex-linked differences in the "circuit diagram" of the brain of human embryos. That's because, as far as I know, essentially nothing is known about this. Brizendine's image of "[watching] a female and a male brain developing via time-lapse photography" so as to "see their circuit diagrams being laid down according to the blueprint drafted by both genes and sex hormones" is a striking metaphor. But it's entirely hypothetical . No one knows what those "circuit diagrams" are, in detail; and in very broad-scale terms, such as the basic layout of sensory and motor cortex, all normal humans are the same. We do know, however, that the details of the brain's circuits develop via a complex interaction between genes and environment, which continues for a long time after birth, and that they are quite different from person to person, independent of sex.
Nevertheless, Brizendine invites us to visualize a sort of embryonic testosterone-poisoning, creating communication-impaired and sexually aggressive males:
A huge testosterone surge beginning in the eighth week will turn this unisex brain male by killing off some cells in the communication centers and growing more cells in the sex and aggression centers.
14: ". . . the sex and aggression centers.": Sur 2005.
This reference resolves to
3. Sur, M., and J.L. Rubenstein (2005) "Patterning and plasticity of the cerebral cortex." Science 310 (5749):805-10.
which is a fascinating review of recent research on cortical development:
The degree to which our genetic endowment (nature) versus our experiences (nurture) mold the development and function of our brains has been the subject of robust discussion and experimental investigation. Research before 1990 led to two general hypotheses: the Protomap (1) and the Protocortex (2). In their most extreme interpretations, the former postulated that the cortical progenitor zone contains the information that generates cortical areas, whereas the latter postulated that thalamic afferent axons, through activity-dependent mechanisms, impose cortical areal identity on an otherwise homogeneous cortex. In the intervening 15 years, tremendous strides have been made in understanding cortical development with molecular, genetic, imaging, and electrophysiological approaches. The new evidence indicates that the development of cortical areas involves a rich array of signals, with considerable interplay between mechanisms intrinsic to cortical progenitors and neurons and mechanisms extrinsic to the cortex, including those requiring neural activity.
Sur and Rubenstein conclude that
The Protomap/Protocortex controversy no longer remains: It is clear that the parcellation of the cerebral cortex into discrete processing areas involves an interwoven cascade of developmental events including both intrinsic and extrinsic mechanisms. The field now has the intellectual foundation and tools that will enable it to elucidate more complex features of cortical development, such as the formation of higher order cortical areas and circuits (which are a robust feature of the primate brain) and the lateralization of cortical functions (136). Insights gained from such studies will undoubtedly facilitate understanding of the mechanisms underlying the evolution of neural systems that control cognition and emotion as well as the etiologies of disorders that derail them.
Great stuff! But there's absolutely nothing in this article about sexual differentiation. Specifically, there's no mention whatsoever of the concept that "[a] huge testosterone surge ... will turn this unisex brain male by killing off some cells in the communication centers and growing more cells in the sex and aggression centers". The terms "sex", "male", "female", "testosterone", "Y chromosome" don't occur in this paper; and Brizendine offers no other support for this claim.
Brizendine contrasts the results of fetal testosterone poisoning with the "unperturbed" "female brain" of the talkative and empathetic "fetal girl":
If the testosterone surge doesn't happen, the female brain continues to grow unperturbed. The fetal girl's brain cells sprout more connections in the communications centers and areas that process emotion.
14: ". . . areas that process emotion.": Hill 2006; Herbert 2005; Sun 2005; Witelson 1995; Goldberg 1994.
These five references resolve to:
4. Hill, H., F. Ott, et al. (2006) "Response execution in lexical decision tasks obscures sex-specific lateralization effects in language processing: Evidence from event-related potential measures during word reading." Cereb Cortex 16(7) 978-989.
5. Herbert, M.R., D.A. Ziegler, et al. (2005) "Brain asymmetries in autism and developmental language disorder: A nested whole-brain analysis." Brain 128(1):213-26 .
6. Sun, T., C. Patoine, et al. (2005) "Early asymmetry of gene transcription in embryonic human left and right cerebral cortex." Science 5729:1794-98.
Witelson, S.F. (1995). "Women have greater density of neurons in posterior temporal cortex." J Neurosci 15(5, Pt. 1):3418:28.
7. Goldberg E., K. Podell, et al. (1994) "Cognitive bias, functional cortical geometry and the frontal lobes: laterality, sex and handedness". J Cog Neurosci 6:276-96.
Executive summary: there's nothing in any of these papers to support the view that "[t]he fetal girl's brain cells sprout more connections in the communications centers and areas that process emotion". You can read them for yourself, if you have the needed subscriptions, or go to a library that has them.
In a bit more detail:
Hill et al. 2006 argue that ERP ("event-related potential") measurements of adult word reading are somewhat sexually differentiated, contrary to previous research which "reported a left-lateralized activation in both sexes". They "used a delayed response to separate semantic processing from response selection and execution", and found that the ERP in the semantic processing phase was more lateralized in male than in female subjects. They don't give any information about the effect size or about the amount of within-sex variation, and overall, this tells us nothing about fetal brain development, or for that matter about how many connections adults have in "communications centers and areas that process emotion".
Herbert et al. 2005 "report a whole-brain MRI morphometric survey of asymmetry in children with high-functioning autism and with developmental language disorder (DLD). Subjects included 46 boys of normal intelligence aged 5.7–11.3 years (16 autistic, 15 DLD, 15 controls)." They found that "[t]he larger units of analysis, including the cerebral hemispheres, the major grey and white matter structures and the cortical lobes, showed no asymmetries in autism or DLD and few asymmetries in controls. However, at the level of cortical parcellation units, autism and DLD showed more asymmetry than controls." Note that this study deals only with the differences in brain structure among boys -- autistic, DLD and controls -- and says nothing about sex differences, fetal or otherwise, and especially, nothing to indicate that "[t]he fetal girl's brain cells sprout more connections in the communications centers and areas that process emotion".
Witelson 1995 is discussed in another post ("The main job of the girl brain"). It's about neuron density of various cortical layers in certain language-related temporal-lobe areas, in post-mortem studies of five women and four men with a mean age of 50. There's nothing about frontal-lobe language areas or about "areas that process emotion", Nor is there anything about neural connections, only about the number of cell bodies per unit volume. The results are complicated: the sizes of the areas vary from individual to individual, as do the measured densities of neurons in different layers of different areas. A crude summary of the results, as I read them, would be that volumes and densities tend to vary inversely, so that the total number of cell bodies remains more nearly the same. The most relevant sentence is probably this one: "Due to the small sample size and the homogeneity of the cases studied, generalizability of the results requires replication by other studies."
Sun et al. 2005 is an attempt to understand the embryogenesis of human cortical asymmetry, independent of sex. The authors "studied gene expression levels between the left and right embryonic hemispheres using serial analysis of gene expression", and found "27 differentially expressed genes, which suggests that human cortical asymmetry is accompanied by early, marked transcriptional asymmetries". They analyzed the brains of one female and four male embryos. Nothing whatever about male/female differences is mentioned. Specifically, there is nothing about "[a] huge testosterone surge ... killing off some cells in the communication centers and growing more cells in the sex and aggression centers", nor anything about how "[t]he fetal girl's brain cells sprout more connections in the communications centers and areas that process emotion". In contrast to Brizendine's vision, these authors assume that female and male brains are similar enough that female and male data can be pooled (which is what they do).
Goldberg et al. 1994 looks at the results of administering the so-called "Cognitive Bias Task" (CBT) to a sample of brain-injured adults and normal controls. Here's their abstract:
Examined handedness and the role of the prefrontal cortex (PC) in 2 cognitive operations: those guiding behavior by internal representations and those ensuring an organism's ability to respond to unanticipated environmental conditions. The Cognitive Bias Task (CBT) was developed and administered in right-handed (RH) and non-RH male and female Ss who were either healthy or had acquired PC lesions. Response biases were more context-dependent in healthy RH males and more context-independent in healthy RH females. Frontal lesion effects were asymmetric in males but symmetric in females. Right frontal lesion effects were similar in females and males, but the effect of left frontal lesions was sexually dimorphic. Conversely, the effects of left posterior lesions were similar in females and males, but the effects of right posterior lesions were sexually dimorphic.
This is very interesting stuff, but it has nothing to do with either comunication or emotion, and says nothing about fetal brain development. If you want to learn more, I suggest you try a later paper by the same authors: E. Goldberg and K. Podell, "Adaptive decision making, ecological validity and the frontal lobes", Journal of Clinical and Experimental Neuropsychology, 22(1) 56-68 (2000).
How does this fetal fork in the road affect us? For one thing, because of her larger communication center, this girl will grow up to be more talkative than her brother. Men use about seven thousand words per day. Women use about twenty thousand. For another, it defines our innate biological destiny, coloring the lens through which each of us views and engages the world.
14: ". . . women use about twenty thousand.": Deacon 1997; Garner 1997; Lewis 1997; Pease 1997; Lakoff 1976; Thorne 1983.
The references resolve to these books:
8. Deacon T. (1997) The Symbolic Species: The Co-Evolution of Language and the Brain.
9. Garner A. (1997) Conversationally Speaking: Tested New Ways to Increase Your Personal and Social Effectiveness.
10. Lewis, M. (1997) "Social behavior and language acquisition." In Interactional conversation and the development of language, ed. B. Haslett. New York: Wiley. 313-30.
11. Pease. A. and A. Garner (1997) Talk Language: How to use conversation for profit and pleasure.
12 Lakoff, R. (1976) Language and Women's Place.
13. Thorne, B. (1983) Language, Gender and Society.
The only empirically-testable part of this is the daily word budget stuff, which is also featured in the book's jacket blurb. As discussed in an earlier post ("Sex-linked lexical budgets"), this meme began to appear in works of pop psychology in the early 1990s, which (in all the cases I've been able to track down) assert the difference without citing any empirical support. Garner 1997 and Pease 1997 are works of this type, and other works by Pease definitely include versions of the claim. I've read all the other books on the list, except for Lewis 1997, and I'm pretty certain that nothing can be found in them to support the idea that the "Men use about seven thousand words per day. Women use about twenty thousand". Since writing that post, I've continued to look into this, and what I've found confirms my belief that this like the "Eskimo words for snow" case analyzed by Laura Martin ( "Eskimo Words for Snow: A Case Study in the Genesis and Decay of an Anthropological Example", American Anthropologist, 1986, pp. 418-423), in which an invented statistic mutates and spreads through the literature on a purely ideological basis, without any empirical support at all.
As for Brizendine's reference to Lewis 1997, which her bibliography lists as
10. Lewis, M. (1997) "Social behavior and language acquisition." In Interactional conversation and the development of language, ed. B. Haslett. New York: Wiley. 313-30.
there seems to be some sort of mistake in it. I can't locate any work with the title Interactional conversation and the development of language. There's a 1977 Wiley book by Michael Lewis with a similar title: Interaction, Conversation, and the Development of Language. And there's a 1997 LEA book by Beth Haslett and Wendy Samter, Children Communicating: The First Five Years. Could Brizendine have conflated the two? If you know anything more about this, please tell me.
1. Until eight weeks old, every fetal brain looks female -- female is nature's default gender setting.
2. If you were to watch a female and a male brain developing via time-lapse photography, you would see their circuit diagrams being laid down according to the blueprint drafted by both genes and sex hormones.
3. A huge testosterone surge beginning in the eighth week will turn this unisex brain male by killing off some cells in the communication centers and growing more cells in the sex and aggression centers.
4. If the testosterone surge doesn't happen, the female brain continues to grow unperturbed. The fetal girl's brain cells sprout more connections in the communications centers and areas that process emotion.
5. because of her larger communication center, this girl will grow up to be more talkative than her brother. Men use about seven thousand words per day. Women use about twenty thousand.
#1 and #2 are more or less true -- except that (a) the research Brizendine cites says that there may be some differences in gene expression in neural tissues from the beginning of fetal development, not necessarily mediated by testosterone and AMH; and (b) the brain's "circuit diagrams" are not laid down once and for all as the embryonic brain grows, but continue to develop for a long time after birth (and in some ways throughout life) .
As for the striking claims #3 and #4, none of the references that Brizendine cites in support of this passage provide any empirical support for them at all.
And claim #5 appears to be a scientific urban legend. There's apparently no evidence that it's true, and some reason to think that it's false.
[Links to other recent Language Log posts on the New Biologism in neuroscience can be found here.]
Posted by Mark Liberman at September 4, 2006 10:40 AM