Were the Victorians cleverer than us? The decline in general intelligence estimated from a meta-analysis of the slowing of simple reaction time

The Victorian era was marked by an explosion of innovation and genius, per capita rates of which appear to have declined subsequently. The presence of dysgenic fertility for IQ amongst Western nations, starting in the 19th century, suggests that these trends might be related to declining IQ. This is because high-IQ people are more productive and more creative. We tested the hypothesis that the Victorians were cleverer than modern populations, using high-quality instruments, namely measures of simple visual reaction time in a meta-analytic study. Simple reaction time measures correlate substantially with measures of general intelligence (g) and are considered elementary measures of cognition. In this study we used the data on the secular slowing of simple reaction time described in a meta-analysis of 14 age-matched studies from Western countries conducted between 1889 and 2004 to estimate the decline in g that may have resulted from the presence of dysgenic fertility. Using psychometric meta-analysis we computed the true correlation between simple reaction time and g, yielding a decline of − 1.16 IQ points per decade or − 13.35 IQ points since Victorian times. These findings strongly indicate that with respect to g the Victorians were substantially cleverer than modern Western populations.     

A Jensen effect on dysgenic fertility: An analysis involving the National Longitudinal Survey of Youth

In this study we attempt to determine whether dysgenic fertility is associated with the Jensen effect. This is investigated with respect to a US population representative sample of 8110 individuals from the National Longitudinal Survey of Youth for whom there exists complete data on IQ and fertility. In addition to the general sample, the sample was also broken out by race and sex so as to examine whether or not the Jensen effect manifested amongst different sub-populations. The method of correlated vectors revealed significant Jensen effects in five of the seven samples, and in all cases the effect was in a direction indicating that subtests with higher g-loadings were associated with larger dysgenic fertility gradients. The magnitude of the difference between Spearman’s ρ and Pearson’s r was non-significant in all cases, suggesting that biasing factors were minimally influencing the result. This finding suggests that dysgenesis occurs on the ‘genetic g’ at the heart of the Jensen effect nexus, unlike the Flynn effect, which is ‘hollow’ with respect to g. Finally, the finding is discussed in the context of two converging lines of evidence indicating that genotypic IQ or ‘genetic g’ really has been declining over the last century.     

How fragile is our intellect? Estimating losses in general intelligence due to both selection and mutation accumulation

Two dysgenic models of declining general intelligence have been proposed. The first posits that since the Industrial Revolution those with low g have had a reproductive advantage over those with high g. The second posits that relaxed purifying selection against deleterious mutations in modern populations has led to g declining due to mutation accumulation. Here, a meta-analytic estimate of the decline due to selection is computed across nine US and UK studies, revealing a loss of .39 points per decade (combined N = 202,924). By combining findings from a high-precision study of the effects of paternal age on offspring g with a study of paternal age and offspring de novo mutation numbers, it is proposed that, 70 de novo mutations per familial generation should reduce offspring g by 2.94 points, or .84 points per decade. Combining the selection and mutation accumulation losses yields a potential overall dysgenic loss of 1.23 points per decade, with upper and lower bound values ranging from 1.92 to .53 points per decade. This estimate is close to those from studies employing the secular slowing of simple reaction time as a potential indicator of declining g, consistent with predictions that mutation accumulation may play a role in these findings.     

Is there any evidence of historical slowing of reaction time? No,unless we compare apples and oranges

In this paper, we reconsider a tendency of historical slowing of simple reactions to visual stimuli declared by Woodley et al. (in press). We begin by reconstructing a pendulum similar to that used by Galton and question whether such an instrument could indeed be appropriate for purposes of RT measurement. Next, we screened the other studies used in Woodley's meta-analysis and note the important properties of these studies that make the RTs that they report incomparable to each other. We claim that there is no evidence of the trend of historical increase in RT after these differences between studies are taken into account. Overall, we conclude that any cross-study comparison of RTs is uninformative and cannot provide any evidence for speculating on the topic of historical change in intelligence.     

Adolescent intelligence and socio-economic wealth independently predict adult marital and reproductive behavior

Counter-intuitively, sociobiological and evolutionary theories predict a negative relationship between g and reproduction when applied to modern humans. Although existing research has documented this dysgenic trend, the association between g and socio-economic factors presents a confound that has not systematically been addressed in prior research. Based on a sample of 325,252 individuals drawn from the nationally representative Project Talent database, we examined the unique effects of g and socio-economic wealth, assessed in adolescence, on marital and reproductive behavior over the next 11 years. Results show that both g and socio-economic wealth have unique, independent negative effects on marital and reproductive behavior such that individuals of higher intelligence and higher wealth delay marriage and reproductive longer than those of lower intelligence and wealth. The effect of g was slightly stronger than that of wealth, though for both variables much of their influence was mediated by educational attainment. Consistent with sociobiological theory, these dysgenic effects were stronger among females than males.     

Sex Differences in Simple Visual Reaction Time: A Historical Meta-Analysis

To test the hypothesis that the magnitude of sex differences in simple visual reaction time (RT) has narrowed across time, a meta-analysis was conducted on 72 effect sizes derived from 21 studies (n=15,003) published over a 73-year period. The analysis provided strong evidence for the hypothesized change. In addition, the analysis indicated that the sex difference in RT was on average smaller with non-U.S. samples than with U.S. samples. No relation was found between the magnitude of the sex difference in RT and age or presence vs. absence of a warning signal. Two factors–-participation in fast-action sports and driving–-are proposed as having been responsible for the decrease in the magnitude of the sex differences in simple visual RT across time.     

Psychometric g and mental processing speed on a semantic verification test

The Semantic Verification Test (SVT) consists of brief, simple statements, which are either true or false, about various arrangements of the letters ABC (e.g. B after A). Mean reaction time (RT) for confirming or disconfirming the various statements varies according to their complexity. In independent studies of university students and Navy recruits, RT and other response latency parameters (intraindividual variability and movement time) from SVT performance show significant correlations of about -0.40 with nonspeeded tests of psychometric g. The mean RTs of adults to the various SVT item types are highly related to the mean error rates on these item types when the SVT is taken by elementary school children as a nonspeeded paper-and-pencil test. RT is correlated with the general cognitive ability factor (g) and not with the test-taking speed factor that is found in speeded paper-and-pencil tests. The degree of correlation between RT and psychometric g does not show any regular relationship to differences in the SVT item-type's complexity or difficulty as indicated by mean RT.     

An integrated perspective on the relation between response speed and intelligence

Research in the field of mental chronometry and individual differences has revealed several robust regularities (Jensen, 2006). These include right-skewed response time (RT) distributions, the worst performance rule, correlations with general intelligence (g) that are more pronounced for RT standard deviations (RTSD) than they are for RT means (RTm), an almost perfect linear relation between individual differences in RTSD and RTm, linear Brinley plots, and stronger correlations between g and inspection time (IT) than between g and RTm. Here we show how all these regularities are manifestations of a single underlying relationship, when viewed through the lens of Ratcliff’s diffusion model ( [Ratcliff, 1978] and [Ratcliff et al., 2008] ). The single underlying relationship is between individual differences in general intelligence and individual differences in “drift rate”, which is just the speed of information processing in Ratcliff’s model. We also test and confirm a strong prediction of the diffusion model, namely that the worst performance rule generalizes to phenomena outside of the field of intelligence. Our approach provides an integrative perspective on intelligence findings.     

Reaction time,evoked brain potentials,and psychometric g in the severely retarded

Measurements derived from reaction time (RT), movement time (MT), and an index of neural adaptability (NA) derived from averaged evoked potentials are significantly related to each other as well as to g factor scores extracted from a battery of 15 psychometric tests in a sample of 54 severely retarded adults. The RT-MT and NA variables show a shrunken multiple R of .64 (p < .001) with psychometric g.     

Effect of directional response variables on eye-hand reaction times and decision time

To determine if direction of response affects reaction time, we measured the time for hand response to a visual stimulus, using a sensitive, microprocessor-based testing device to determine simple reaction time (RT), choice RT, and decision time. Mean simple RT was 207 +/- 3.7 msec. (mean +/- SEM); mean choice RT was 268 +/- 4.2 msec; and mean decision time was 61 msec. No differences were noted for leftward versus rightward movements, or midline versus lateral movements. Choice RT increased by 1.49 msec./yr. of age. Simple RT increased significantly with age for the nondominant hand, but not for the dominant hand. Right-handed subjects were more rapid with the dominant hand for choice RT. We conclude that dominance of hand tested and test initiation mechanism have major effects, but direction of movement in the lateral plane has little effect on reaction time.     

The cultural mediation hypothesis: A critical examination

A fascinating hypothesis has been proposed by Woodley (2010, Intelligence, 38: 471–480) to explain why intelligence, in contemporary Western countries, appears to be positively associated with left wing political attitudes. According to the hypothesis, the current Western ‘norm’ is relatively left-wing values, the intelligent are better at norm-mapping and better at perceiving the benefits of conforming to the dominant set of values and this explains why they are more likely, in this context, to be relatively left wing. This article will critically examine this hypothesis. It will argue that the evidence for it is questionable and open to different interpretations, there is evidence that contradicts it, and it leaves key questions unanswered. The article will propose that a simpler explanation for the association is that intelligence is positively associated with openness-intellect, low time preference and agreeableness.     

Partial advance information,number of alternatives,and motor processes: an electromyographic study

We present evidence that advance information reducing the number of stimulus-response alternatives in a choice reaction time (RT) task can shorten the very latest motoric stages of RT. Effects of such advance information on late stages of RT have been demonstrated recently by Osman, Moore, and Ulrich (Acta Psychol. 90 (1995) 111), Leuthold, Sommer, and Ulrich (J. Exp. Psychol: Gen. 125 (1996) 307), Müller-Gethmann, Rinkenauer, Stahl, and Ulrich (Psychophysiology 37 (2000) 507). These studies found that advance information shortens the portion of the RT interval following onset of a movement-related brain potential (lateralized readiness potential). Osman et al. and Müller-Gethmann et al. also examined the portion of the RT interval following the start of electromyographic (EMG) activity and found no effect of advance information. Based on Osman et al.'s null result, Leuthold et al. speculated that advance information may shorten only the RT stages preceding EMG activity. This conclusion, however, is questionable because of limitations in the EMG measures employed by both Osman et al. and Müller-Gethmann et al. We have reanalyzed the results of a previously reported experiment (Acta Psychol. 101 (1999) 243) to show that advance information can in fact affect the rate of recruitment of motor units in the prime mover of the responding limb.     

Reaction time,movement time,and intelligence

Speed of information processing is measured in terms of reaction time (RT) and movement time (MT) to five stimulus displays which differ in the amount of information transmitted, over a range from 0 to 3 bits of information. RT, but not MT, increases as a linear function of the number of bits in the stimulus display. RT and MT show reliable individual differences which are significantly correlated with intelligence as measured by Raven's Standard Progressive Matrices.     

Structural brain variation,age, and response time

Response time (RT) generally slows with aging, but the contribution of structural brain changes to this slowing is unknown. We used voxel-based morphometry (VBM) to determine gray matter (GM) and white matter (WM) brain volumes in 9 middle-aged adults (38–58 years old) and 9 seniors (66–82 years old). We correlated brain volumes with RT assessed in both a simple visual stimulus-response task and a visual continuous recognition memory task. No GM correlations with simple RT were significant; there was one WM correlation in the right fusiform gyrus. In the memory task, faster RT was correlated (p<.05, corrected) with less GM in the globus pallidus, the parahippocampus, and the thalamus for both groups. Several Brodmann areas (BA) differed between the groups such that in each area, less GM was correlated with slower RTs in the middle-aged group but with faster RTs in the senior group (BAs 19, 37, 46, 9, 8, 6, 13, 10, 41, and 7). The results suggest that individual differences in specific brain structure volumes should be considered as potential moderating factors in cognitive brain imaging studies.     

The influence of catch trial frequency on simple reaction time

The major purpose of the present investigation was to examine the combined influence of variations in foreperiod length (1.0, 5.0 sec) and catch trial frequency (0.0, 0.1, 0.2) on simple reaction time (RT) magnitude when verbal mediation was permitted and when it was not. A significant foreperiod length, catch trial frequency, mediation type interaction was obtained supporting the notion that all three factors influence a common process (i.e. Sternberg 1969), here called ‘preparation’. More specifically, for the non-mediation group, foreperiod length and catch trial frequency had both additive (0.0 vs 0.1) and interactive (0.1 vs 0.2) effects on mean simple RT while only catch trial frequency significantly altered simple RT for the mediating group. In all instances, mean simple RT increased significantly when some catch trials were employed as opposed to when none were used, a finding which was interpreted as reflecting an induced preparation decrement upon the introduction of catch trials.     

Orthogonal factors of mental ability? A response to Hampshire et al.

We respond to the main points raised by Hampshire et al. (2014). First, we note that the simulation study of Hampshire et al. (2012) was based on the assumption that individual differences in the capacities of the three proposed brain networks are mutually independent. However, this orthogonality cannot be assumed even if those networks are anatomically and functionally distinct. We also consider further the implications of our finding that a blended variable or task-mixing model of the structure of mental ability would require nearly every mental ability task to load on nearly every factor of mental ability. We explain that such a result is implausible and favors more parsimonious models that include a g factor.     

Temporal, but not spatial, context modulates a masked prime's effect on temporal order judgment, but not on response latency

Three experiments investigated a dissociation originally described by Neumann, Esselmann, and Klotz. Stimuli were geometric shapes, preceded by similar shapes that were masked by metacontrast. Each experiment consisted of three parts. In the reaction time (RT) part, participants saw an array of geometric shapes, one of which was marked by bars, and had to respond to the marked shape's position by pressing an appropriate button. A prime (a similar, but smaller stimulus) preceded either the marked or an unmarked stimulus. In the temporal order judgment (TOJ) part, the task was to judge the temporal order of the marked and the unmarked stimulus. In the detection part, detectability of the prime was tested. Although its detectability was zero or close to zero, the prime affected both RT and the apparent onset as measured by TOJ. The effect on RT was significantly larger than the effect on TOJ (Exp. 1). Increasing the spatial context (number of non-target stimuli in the display) did not affect this pattern (Exp. 2). By contrast, reducing the temporal context (range of stimulus onset asynchronies) abolished the prime's effect in the TOJ task, although the prime affected RT under identical conditions. It is concluded that partially different mechanisms mediate the prime's effect in the two tasks and that the effect of stimulus context on TOJ found in the Neumann et al. study was due to temporal, not spatial context. Received: 17 December 1997 / Accepted: 15 December 1998     

The dependability of the general factor of intelligence: Why small, single-factor models do not adequately represent g

Floyd, Shands, Rafael, Bergeron and McGrew (2009) used generalizability theory to test the reliability of general-factor loadings and to compare three different sources of error in them: the test battery size, the test battery composition, the factor-extraction technique, and their interactions. They found that their general-factor loadings were moderately to strongly dependable. We replicated the methods of Floyd et al. (2009) in a different sample of tests, from the Minnesota Study of Twins Reared Apart (MISTRA). Our first hypothesis was that, given the greater diversity of the tests in MISTRA, the general-factor loadings would be less dependable than in Floyd et al. (2009). Our second hypothesis, contrary to the positions of Floyd et al. (2009) and Jensen and Weng (1994), was that the general factors from the small, randomly-formed test batteries would differ substantively from the general factor from a well-specified hierarchical model of all available tests. Subtests from MISTRA were randomly selected to form independent and overlapping batteries of 2, 4 and 8 tests in size, and the general-factor loadings of eight probe tests were obtained in each battery by principal components analysis, principal factor analysis and maximum likelihood estimation. Results initially indicated that the general-factor loadings were unexpectedly more dependable than in Floyd et al. (2009); however, further analysis revealed that this was due to the greater diversity of our probe tests. After adjustment for this difference in diversity, and consideration of the representativeness of our probe tests versus those of Floyd et al. (2009), our first hypothesis of lower dependability was confirmed in the overlapping batteries, but not the independent ones. To test the second hypothesis, we correlated g factor scores from the random test batteries with g factor scores from the VPR model; we also calculated special coefficients of congruence on the same relation. Consistent with our second hypothesis, the general factors from small non-hierarchical models were found to not be reliable enough for the purposes of theoretical research. We discuss appropriate standards for the construction and factor analysis of intelligence test batteries.     

Comparing subtest profiles of g loadings and correlations with RT measures

It is argued that if a response time (RT) measure correlates with psychometric test (PT) scores because it shares variance in common with general intelligence, g, then the profile of g loadings for a set of PTs would be predictable from the profile of correlations between the RT measure and the PT scores. On the other hand, if an RT correlates with PTs through variance not shared with g, the g loadings of the PTs should be unrelated to the correlations between RT and PTs. The profiles of g loadings and correlations with RT were compared for a set of 6 RT measures and 8 PT scores from Smith and Stanley (1983). To demonstrate statistical significance, standard errors of the statistics were generated by Efron's bootstrap technique (Efron, 1979). It was clearly shown that the profile of the PT's g loadings could be well predicted from the RT-PT correlations for four of the RT measures. It can be concluded that RT tasks do measure general intelligence. Analysis of the errors in prediction suggested that the RTs may correlate more with fluid than crystallized intelligence.     

Subjective responses to alcoholic and non-alcoholic beverages by alcoholic respondents

Changes in the subjective response to alcohol following electrical aversive conditioning have been studied through the use of the semantic differential (Costello, 1974; Costello et al., 1974a, 1974b; Hallam et al., 1972; Miller et al., 1974; Miller et al. 1973). As pointed out by the Costello group and by Kaplan (1972) there are several problems with this technique. The midpoint of the 7-point scale may be psychologically impossible to define if the subject does not perceive the adjectives pairs as mutually antagonistic. It is frequently assumed that the adjective pairs are bipolar, based on the original work of Osgood et al. (1957). Recent evidence (Costello, 1974; Costello et al., 1974a, 1974b) has demonstrated that the scales used by Hallam et al. (1972) were in fact, not bipolar.The stimuli used by Costello and coworkers were an alcoholic and a non-alcoholic slide, following the work of Hallam et al. (1972). Recent studies (Miller et al., 1974; Miller et al., 1973; Wilson, 1973) have utilized rating the taste of real alcohol, as opposed to slides, in order to measure the effectiveness of behavioral therapies in alcoholism. The subject is asked to taste several beverages, some of which have alcohol, and rate the taste of each drink on a set of semantic differential adjective pairs. Miller et al. (1974), found that while the actual amount of pre-therapy alcoholic beverage consumed predicted which subjects did better at 6 months outcome, the semantic differential ratings did not. One reason might have been that the scales were not bipolar and thus the semantic measurement was highly unreliable.The purpose of the present study was to replicate the Costello studies, using real beverages, alcoholic and non-alcoholic as the stimuli.