Dissolution of the social contract

I've been meaning to point to a very cogent essay by Marmor and Mashaw on why conditions for recovery from the great depression of the early 1930's were more propitious than those that prevail in our current recession - which promises to be very prolonged.

...there is a crucial difference between then and now: the words that our political leaders use to talk about our problems have changed. Where politicians once drew on a morally resonant language of people, family and shared social concern, they now deploy the cold technical idiom of budgetary accounting...This is more than a superficial difference in rhetoric. It threatens to deprive us of the intellectual resources needed to address today’s problems.

From the 1930s to the 1960s...American public discourse was filled with references to the social circumstances of average citizens, our common institutions and our common history. Over the last five decades, that discourse has changed in ways that emphasize individual choice, agency and preferences. The language of sociology and common culture has been replaced by the language of economics and individualism.

In 1934, the government was us. We had shared circumstances, shared risks and shared obligations. Today the government is the other — not an institution for the achievement of our common goals, but an alien presence that stands between us and the realization of individual ambitions. Programs of social insurance have become “entitlements,” a word apparently meant to signify not a collectively provided and cherished basis for family-income security, but a sinister threat to our national well-being.

Over the last 50 years we seem to have lost the words — and with them the ideas — to frame our situation appropriately.

Differences in reality monitoring correlate with prefrontal cortex variations

Here is an intriguing bit of work from Buda et al.:

Much recent interest has centered on understanding the relationship between brain structure variability and individual differences in cognition, but there has been little progress in identifying specific neuroanatomical bases of such individual differences. One cognitive ability that exhibits considerable variability in the healthy population is reality monitoring; the cognitive processes used to introspectively judge whether a memory came from an internal or external source (e.g., whether an event was imagined or actually occurred). Neuroimaging research has implicated the medial anterior prefrontal cortex (PFC) in reality monitoring, and here we sought to determine whether morphological variability in a specific anteromedial PFC brain structure, the paracingulate sulcus (PCS), might underlie performance. Fifty-three healthy volunteers were selected on the basis of MRI scans and classified into four groups according to presence or absence of the PCS in their left or right hemisphere. The group with absence of the PCS in both hemispheres showed significantly reduced reality monitoring performance and ability to introspect metacognitively about their performance when compared with other participants. Consistent with the prediction that sulcal absence might mean greater volume in the surrounding frontal gyri, voxel-based morphometry revealed a significant negative correlation between anterior PFC gray matter and reality monitoring performance. The findings provide evidence that individual differences in introspective abilities like reality monitoring may be associated with specific structural variability in the PFC.

Examples of prominent (left) and absent (right) PCS classifications. In the left panel, PCS is indicated by the red arrow.

Merging emotional information from voice and faces in the brain.

Klasen et al. find the ventral posterior cingulate to be a central structure for supramodal representation of complex emotional information. The left amygdala reflects input of happy stimuli from multiple sensory inputs:

Supramodal representation of emotion and its neural substrates have recently attracted attention as a marker of social cognition. However, the question whether perceptual integration of facial and vocal emotions takes place in primary sensory areas, multimodal cortices, or in affective structures remains unanswered yet. Using novel computer-generated stimuli, we combined emotional faces and voices in congruent and incongruent ways and assessed functional brain data (fMRI) during an emotional classification task. Both congruent and incongruent audiovisual stimuli evoked larger responses in thalamus and superior temporal regions compared with unimodal conditions. Congruent emotions were characterized by activation in amygdala, insula, ventral posterior cingulate (vPCC), temporo-occipital, and auditory cortices; incongruent emotions activated a frontoparietal network and bilateral caudate nucleus, indicating a greater processing load in working memory and emotion-encoding areas. The vPCC alone exhibited differential reactions to congruency and incongruency for all emotion categories and can thus be considered a central structure for supramodal representation of complex emotional information. Moreover, the left amygdala reflected supramodal representation of happy stimuli. These findings document that emotional information does not merge at the perceptual audiovisual integration level in unimodal or multimodal areas, but in vPCC and amygdala.

Measuring Zeitgeist from the Tweet stream.

Golder and Macy have analyzed more than 509 million Twitter posts by 2.4 million users over a 2-year period in order to study collective mood (using Twitter to track the mood of nations sort of like using satellites to track the state of the atmosphere!). They used a freely available protocol provided by Twitter to download tweets originating from 84 countries between February 2008 and January 2010, and searched these messages for roughly 1000 words on a tried-and-tested list of words associated with positive (agree, fantastic, super) and negative (afraid, mad, panic) emotion. Despite very different cultures, geographies, and religions, the U.S., Canada, the UK, Australia, India, and English-speaking Africa all showed similar mood rhythms:

We identified individual-level diurnal and seasonal mood rhythms in cultures across the globe, using data from millions of public Twitter messages. We found that individuals awaken in a good mood that deteriorates as the day progresses—which is consistent with the effects of sleep and circadian rhythm—and that seasonal change in baseline positive affect varies with change in daylength. People are happier on weekends, but the morning peak in positive affect is delayed by 2 hours, which suggests that people awaken later on weekends.

A graphic from the article:

Hourly changes in individual affect broken down by day of the week (top, positive affect, PA; bottom,  negative affect, NA). Each series shows mean affect (black lines) and 95% confidence interval (colored regions). (Experimental psychologists have repeatedly demonstrated that positive and negative affect are independent dimensions. Positive affect (PA) includes enthusiasm, delight, activeness, and alertness, whereas negative affect (NA) includes distress, fear, anger, guilt, and disgust)

Fatherhood decreases testosterone.

Gettler et al. find that, in a community-based sample from the Philippines, men with higher testosterone level are more likely to marry than men with lower testosterone; that men who marry and become fathers experience declines in testosterone; and that men who provide more paternal care have lower testosterone levels than fathers who provide less care:

In species in which males care for young, testosterone (T) is often high during mating periods but then declines to allow for caregiving of resulting offspring. This model may apply to human males, but past human studies of T and fatherhood have been cross-sectional, making it unclear whether fatherhood suppresses T or if men with lower T are more likely to become fathers. Here, we use a large representative study in the Philippines (n = 624) to show that among single nonfathers at baseline (2005) (21.5 ± 0.3 y), men with high waking T were more likely to become partnered fathers by the time of follow-up 4.5 y later (P < 0.05). Men who became partnered fathers then experienced large declines in waking (median: −26%) and evening (median: −34%) T, which were significantly greater than declines in single nonfathers (P < 0.001). Consistent with the hypothesis that child interaction suppresses T, fathers reporting 3 h or more of daily childcare had lower T at follow-up compared with fathers not involved in care (P < 0.05). Using longitudinal data, these findings show that T and reproductive strategy have bidirectional relationships in human males, with high T predicting subsequent mating success but then declining rapidly after men become fathers. Our findings suggest that T mediates tradeoffs between mating and parenting in humans, as seen in other species in which fathers care for young. They also highlight one likely explanation for previously observed health disparities between partnered fathers and single men.

Mimicry can foster both rapport and threat.

Liu does experiments that illustrate two aspects of mimicry. Mimicry can bond people by fostering rapport and liking, but appears to have the opposite effect if participants in an interaction are primed by reminders of money. Money-primed participants liked a mimicking interaction partner less than they liked a nonmimicking partner, an effect that appears to have been due to enhanced feelings of threat. Their observations are consonant with studies that have shown that reminders of money elicit a self-sufficient state characterized by two tendencies: First, eagerness to pursue personal goals and freedom, persisting longer than others on difficult tasks and hesitating to ask for help; and second, acting more insensitive to others, desiring solo versus activities and showing more indifference to social exclusion.

The Jekyll and Hyde of emotional intelligence

Côté et al. make some observations about emotional smarts:

Does emotional intelligence promote behavior that strictly benefits the greater good, or can it also advance interpersonal deviance? In the investigation reported here, we tested the possibility that a core facet of emotional intelligence—emotion-regulation knowledge—can promote both prosocial and interpersonally deviant behavior. Drawing from research on how the effective regulation of emotion promotes goal achievement, we predicted that emotion-regulation knowledge would strengthen the effects of other-oriented and self-oriented personality traits on prosocial behavior and interpersonal deviance, respectively. Among individuals with higher emotion-regulation knowledge, a first study noted that moral identity exhibited a stronger positive association with prosocial behavior in a social dilemma [i.e. the study confirmed the authors' prediction that there is an association between moral identity and prosocial behavior in a social dilemma, this association being stronger among individuals with high emotion-regulation knowledge]. A second study found that the positive relation between Machiavellianism and interpersonal deviance was stronger when emotion-regulation knowledge was high rather than when it was low, thus pointing out the dark side of emotion-regulation knowledge.

How our brain links to our immune system via vagus nerve

A large number of studies show that our psychological state can influence our immune system. Social isolation and stress weaken our immune system, and social affiliation and calm can strengthen it (the latter are associated with increased activity of the vagus nerve, which runs from the brainstem to many parts of the body). Now a multinational collaboration headed by Kevin Tracey has shown that stimulating the vagus nerve halts the pumping out of inflammatory signals by the immune system, with the neurotransmitter molecular responsible being acetyl choline - but, the Vagus nerve is not releasing it directly. Instead the splenic branch of the Vagus nerve is releasing noradrenaline in the spleen whose white blood cells (CD4 lymphocytes) are then secreting the acetyl choline which switches off production of inflammatory chemical by nearby cells. Here is the abstract:

Neural circuits regulate cytokine production to prevent potentially damaging inflammation. A prototypical vagus nerve circuit, the inflammatory reflex, inhibits tumor necrosis factor–α production in spleen by a mechanism requiring acetylcholine signaling through the  alpha 7 nicotinic acetylcholine receptor expressed on cytokine-producing macrophages. Nerve fibers in spleen lack the enzymatic machinery necessary for acetylcholine production; therefore, how does this neural circuit terminate in cholinergic signaling? We identified an acetylcholine-producing, memory phenotype T cell population in mice that is integral to the inflammatory reflex. These acetylcholine-producing T cells were required for the inhibition of cytokine production by vagus nerve stimulation. Thus, action potentials originating in the vagus nerve regulate T cells, which in turn produce the neurotransmitter acetylcholine required to control innate immune responses.

Be Happy! ... regulate your own amygdala

Not really....the conditions used in the experiments by Zotev et al.et al. - having a handy fMRI machine nearby - are not exactly accessible to most of us. Because it is known that activation of the left amygdala is associated with happy emotions, they tried a biofeedback trick:

We investigated the feasibility of training healthy humans to self-regulate the hemodynamic activity of the amygdala, which plays major roles in emotional processing. Participants in the experimental group were provided with ongoing information about the blood oxygen level dependent (BOLD) activity in the left amygdala (LA) and were instructed to raise the BOLD rtfMRI signal by contemplating positive autobiographical memories. A control group was assigned the same task but was instead provided with sham feedback from the left horizontal segment of the intraparietal sulcus (HIPS) region. In the LA, we found a significant BOLD signal increase due to rtfMRI neurofeedback training in the experimental group versus the control group...The whole brain data analysis revealed significant differences for Happy Memories versus Rest condition between the experimental and control groups...The findings demonstrate that healthy subjects can learn to regulate their amygdala activation using rtfMRI neurofeedback, suggesting possible applications of rtfMRI neurofeedback training in the treatment of patients with neuropsychiatric disorders.

Psychological resources correlate with an Oxytocin receptor gene.

Interesting work from Saphire-Bernstein et al.. An SNP is a single nucleotide polymorphism, in this case a switch at a particular site between the base adenine (A) and guanine (G) in the DNA coding for an oxytocin receptor named OXTR. Every person carries two copies of this gene (one from each parent), and the finding is that A/A or A/G individuals are less psychologically robust than G/G individuals. This suggests a strong relationship between OXTR and psychological resources. Here is the abstract:

Psychological resources—optimism, mastery, and self-esteem—buffer the deleterious effects of stress and are predictors of neurophysiological and psychological health-related outcomes. These resources have been shown to be highly heritable, yet the genetic basis for this heritability remains unknown. Here, we report a link between the oxytocin receptor (OXTR) SNP rs53576 and psychological resources, such that carriers of the “A” allele have lower levels of optimism, mastery, and self-esteem, relative to G/G homozygotes. OXTR was also associated with depressive symptomatology. Mediation analysis indicates that the effects of OXTR on depressive symptoms may be largely mediated by the influence of OXTR on psychological resources.

Income inequality, happiness, and trust.

From Oishi et al., observations that correlate with many experiments showing that people are more concerned with their relative income rank and fairness, rather than absolute income:

Using General Social Survey data from 1972 to 2008, we found that Americans were on average happier in the years with less national income inequality than in the years with more national income inequality. We further demonstrated that this inverse relation between income inequality and happiness was explained by perceived fairness and general trust. That is, Americans trusted other people less and perceived other people to be less fair in the years with more national income inequality than in the years with less national income inequality. The negative association between income inequality and happiness held for lower-income respondents, but not for higher-income respondents. Most important, we found that the negative link between income inequality and the happiness of lower-income respondents was explained not by lower household income, but by perceived unfairness and lack of trust.

Think away colds, and...Less fat, less depression

My thanks to a colleague (Ken Munkres, Emeritus Professor of Molecular Biology and Genetics at UW Madison) who sent me these two tidbits:

How to Think Away Those Cold Symptoms
Tinker Bell was right all along

Balancing Your Fat Intake Controls Depression
The intersection of diet and psychiatry

There is nothing paranormal about near-death experiences.

Mobbs and Watt argue that neuroscience can explain seeing bright lights, meeting the dead, or being convinced you are one of them, suggesting that there is nothing paranormal about these experiences. Instead, near-death experiences are the manifestation of normal brain function gone awry, during a traumatic, and sometimes harmless, event. They engage several basic features of near death experiences (awareness of being dead, out of body experiences, tunnel of light, meeting deceased people, euphoric emotions) and note the brain regions whose perturbation by electrical stimulation or ischemic strokes can induce each phenomenon. Some can be observed in normal individuals under particular conditions of sensory stimulation (I've done posts on the Blanke experiments on out of body experiences). Their arguments are of the "may be" or "could be" sort, and their summary seems to to be particularly vague hand waving...

The near-death experience is a complex set of phenomena and a single account will not capture all its components. One recent theory is that the basic arousal systems beginning in the midbrain may account for many of the components of the near-death experience. Of interest is the locus coeruleus, a midbrain region involved in the release of noradrenaline. Noradrenaline is known to be involved in arousal related to fear, stress, and hypercarbia, and is highly connected to regions that mediate emotion and memory, including the amygdala and hippocampus. Indeed, stimulation of the noradrenaline system has been shown to enhance and consolidate memory, and plays a critical role in the sleep-wake cycle, including REM sleep. Along with basic midbrain systems, such as the periaqueductal gray, a region involved in opioid analgesia and basic fear responses, and the ventral tegmental area, which is a core dopamine reward area, the noradrenaline system may be part of a basic set of systems that directly or indirectly evoke positive emotions, hallucinations and other features of the near-death experience.

Taken together, the scientific evidence suggests that all aspects of the near-death experience have a neurophysiological or psychological basis: the vivid pleasure frequently experienced in near-death experiences may be the result of fear-elicited opioid release, while the life review and REM components of the near-death experience could be attributed to the action of the locus coeruleus- noradrenaline system. Out-of-body experiences and feelings of disconnection with the physical body could arise because of a breakdown in multisensory processes, and the bright lights and tunneling could be the result of a peripheral to fovea breakdown of the visual system through oxygen deprivation. A priori expectations, where the individual makes sense of the situation by believing they will experience the archetypal near-death experience package, may also play a crucial role. If one challenge of science is to demystify the world, then research should begin to test these and other hypotheses. Only then will discussion of near-death experiences move beyond theological dialogue and into the lawful realm of empirical neurobiology.

Longevity gene debate.

A collaboration between German, French, and British laboratories is challenging the basic observations on extending life span in nematodes and fruitflies, made in American laboraties, that started off the excitement about resveratrol (in red wine) and the sirtuin genes. From Wade's review:

Both experiments were flawed, they say, because the worms and flies used as a control were not genetically identical to the test organisms. The London researchers report that they have repeated the experiments with proper controls and found that extra sirtuin does not, after all, make the worms or flies live longer.

Yoghurt lowers stress in the brain.

Here is a totally fascinating study done at the University College Cork indicating that the presence of probiotic bacteria in the gut can result in lower stress in the brain (via a pathway involving the vagus nerve and the brain neurotransmitter GABA). Perhaps that’s one reason for the longevity observed in many yogurt-eating populations. Their

...findings highlight the important role of bacteria in the bidirectional communication of the gut–brain axis and suggest that certain organisms may prove to be useful therapeutic adjuncts in stress-related disorders such as anxiety and depression.

I've started having organic yoghurt with mixes of different probiotic bacteria such as the Lactobacillus mentioned below for lunch (not the usual supermarket stuff larded with high fructose corn syrup), and of course am feeling more calm already (placebo effect, anyone?). Here is the whole abstract that notes the brain chemistry involved:

There is increasing, but largely indirect, evidence pointing to an effect of commensal gut microbiota on the central nervous system (CNS). However, it is unknown whether lactic acid bacteria such as Lactobacillus rhamnosus could have a direct effect on neurotransmitter receptors in the CNS in normal, healthy animals. GABA is the main CNS inhibitory neurotransmitter and is significantly involved in regulating many physiological and psychological processes. Alterations in central GABA receptor expression are implicated in the pathogenesis of anxiety and depression, which are highly comorbid with functional bowel disorders. In this work, we show that chronic treatment with L. rhamnosus (JB-1) induced region-dependent alterations in GABAB1b mRNA in the brain with increases in cortical regions (cingulate and prelimbic) and concomitant reductions in expression in the hippocampus, amygdala, and locus coeruleus, in comparison with control-fed mice. In addition, L. rhamnosus (JB-1) reduced GABAAα2 mRNA expression in the prefrontal cortex and amygdala, but increased GABAAα2 in the hippocampus. Importantly, L. rhamnosus (JB-1) reduced stress-induced corticosterone and anxiety- and depression-related behavior. Moreover, the neurochemical and behavioral effects were not found in vagotomized mice, identifying the vagus as a major modulatory constitutive communication pathway between the bacteria exposed to the gut and the brain. Together, these findings highlight the important role of bacteria in the bidirectional communication of the gut–brain axis and suggest that certain organisms may prove to be useful therapeutic adjuncts in stress-related disorders such as anxiety and depression.

Infidelity yields better offspring

Here is a study of birds (dark-eyed juncos) who form a social pair with one partner but produce more than one-quarter of their offspring from extra-pair matings. These young go on to reproduce more successfully than those sired within the social pair. (I could wonder if this is relevant to us humans.)

In many species, each female pairs with a single male for the purpose of rearing offspring, but may also engage in extra-pair copulations. Despite the prevalence of such promiscuity, whether and how multiple mating benefits females remains an open question. Multiple mating is typically thought to be favoured primarily through indirect benefits (i.e. heritable effects on the fitness of offspring). This prediction has been repeatedly tested in a variety of species, but the evidence has been equivocal, perhaps because such studies have focused on pre-reproductive survival rather than lifetime fitness of offspring. Here, we show that in a songbird, the dark-eyed junco (Junco hyemalis), both male and female offspring produced by extra-pair fertilizations have higher lifetime reproductive success than do offspring sired within the social pair. Furthermore, adult male offspring sired via extra-pair matings are more likely to sire extra-pair offspring (EPO) themselves, suggesting that fitness benefits to males accrue primarily through enhanced mating success. By contrast, female EPO benefited primarily through enhanced fecundity. Our results provide strong support for the hypothesis that the evolution of extra-pair mating by females is favored by indirect benefits and shows that such benefits accrue much later in the offspring's life than previously documented.

Atul Gawande on improving skilled performance by coaching

Anything Gawande writes is worth reading. I have previously noted his essays on aging and medical practice. In his most recent New Yorker essay, he discusses how expert coaching is used in many fields, and describes his own experience with recruiting a senior surgeon to improve his own experience as a surgeon.

The Buddha's Biology

I had started to occasionally re-post an old post that really struck me during my scan of this blog since its beginning in 2006, and now repeat the following old post, having come upon it for a second time:

I want to mention a book by that I have found to be a useful summary and distillation of correspondences between classical Buddhist psychology and modern psychology and evolutionary biology. Don't let its self-helpy new-agey title put you off (Buddha's Nature: A Practical Guide to Discovering Your Place in the Cosmos). It's by a crazy guy named Wes Nisker, a stand up Buddhist comic and veteran of the sixties and seventies new age San Francisco scene whose other writings include "The Big Bang, The Buddha, and the Baby Boom" and "The Essential Crazy Wisdom". It is a largely accurate descriptions of how Buddhism's four foundations of mindfulness can be taken to correspond to the bottom-up construction of our nervous system and consciousness, and to stages in the evolution of our nervous systems.

Sensing or exploring the nature of our elemental physical existence, our body breathing and homeostasis, is a focus of the Buddha's First Foundation of Mindfulness. This first foundation corresponds to physical elements of the body and homeostasis (regulation of blood flow, body temperature, etc.) These functions center in primitive brain stem structures we share with reptiles and other vertebrates. This core regulates interactions with the physical world elemental to having a self that we seldom think about - like breathing, supporting ourselves against gravity, seeing, tasting, smelling, touching, hearing.

These core structures also regulate our urge to remedy hunger, to have sex, to approach or avoid, to flee or fight when suddenly presented with very threatening situations. Our experience of these primary and instinctual basic drives, in its urgency and automaticity, has a very different quality than our experience of thoughts or more complicated emotions. The Buddha's Second Foundation of Mindfulness rests on the sentience of the nervous system which can note these elemental feelings, impressions of pleasant/unpleasant/neutral/painful, etc. We can, in more quiet moments of reflection or meditation note the more muted `flickers' of these primal forces, appearing and disappearing almost as transient quantal energies.

Our human introspective access to, observation of, emotional feelings more nuanced than the basic drives mentioned above is the focus of the Buddha's third foundation of mindfulness (affection, fear, anger, sadness, playfulness, etc.). These are regulated by a new kind of cortex that appears in mammals between the brain stem and the outer layer of the cortex, usually referred to as the limbic system.

Finally, our higher level cognitive abilities associated with the newer cortex (neocortex) that forms the top layers or our brain - our ability to note how thoughts and feelings are produced, as natural occurrences like breathing or the heartbeat - are a focus of the Buddha's fourth foundation of mindfulness.

Nisker's book has several sections of exercises or meditations useful in sensing layers of the self, its evolutionary nature, and its symbiosis with the external social and physical world.

Delayed gratification - 40 years later.

Casey et al. (open access) do a followup of the famous "marshmallow experiments" that showed young children who are better at delaying gratification to obtain a greater reward do better latter in life. They were able to test 60 individuals from the original study, now in their mid-40s, and in a subset of these were able to demonstrate stable differences in frontostriatal circuitries that integrate motivational and control processes in low delayers versus high delayers.

We examined the neural basis of self-regulation in individuals from a cohort of preschoolers who performed the delay-of-gratification task 4 decades ago. Nearly 60 individuals, now in their mid-forties, were tested on “hot” and “cool” versions of a go/nogo task to assess whether delay of gratification in childhood predicts impulse control abilities and sensitivity to alluring cues (happy faces). Individuals who were less able to delay gratification in preschool and consistently showed low self-control abilities in their twenties and thirties performed more poorly than did high delayers when having to suppress a response to a happy face but not to a neutral or fearful face. This finding suggests that sensitivity to environmental hot cues plays a significant role in individuals’ ability to suppress actions toward such stimuli. A subset of these participants (n = 26) underwent functional imaging for the first time to test for biased recruitment of frontostriatal circuitry when required to suppress responses to alluring cues. Whereas the prefrontal cortex differentiated between nogo and go trials to a greater extent in high delayers, the ventral striatum showed exaggerated recruitment in low delayers. Thus, resistance to temptation as measured originally by the delay-of-gratification task is a relatively stable individual difference that predicts reliable biases in frontostriatal circuitries that integrate motivational and control processes.

Nurture affects gender differences in spatial abilities

Here is a fascinating bit from Hoffman et al.:

Women remain significantly underrepresented in the science, engineering, and technology workforce. Some have argued that spatial ability differences, which represent the most persistent gender differences in the cognitive literature, are partly responsible for this gap. The underlying forces at work shaping the observed spatial ability differences revolve naturally around the relative roles of nature and nurture. Although these forces remain among the most hotly debated in all of the sciences, the evidence for nurture is tenuous, because it is difficult to compare gender differences among biologically similar groups with distinct nurture. In this study, we use a large-scale incentivized experiment with nearly 1,300 participants to show that the gender gap in spatial abilities, measured by time to solve a puzzle, disappears when we move from a patrilineal society to an adjoining matrilineal society. We also show that about one-third of the effect can be explained by differences in education. Given that none of our participants have experience with puzzle solving and that villagers from both societies have the same means of subsistence and shared genetic background, we argue that these results show the role of nurture in the gender gap in cognitive abilities.