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There is some meta-analytic evidence that stimulants increase the IQ test scores of ADHD children by 2 to 7 points (Jepsen et al., 2009). Although giving stimulants to non-ADHD children may be unethical, I'm still wondering if there is improvement in IQ test score in children (or adults) who do not meet the criteria for an ADHD diagnosis. An additional motivation for asking this is that a study by Tsai et al. (2013) found that the IQ test score improvement from methylphenidate "had no correlation with the decrement of ADHD symptoms."

Jepsen, J. R. M., Fagerlund, B., & Mortensen, E. L. (2009). Do attention deficits influence IQ assessment in children and adolescents with ADHD?. Journal of Attention Disorders, 12(6), 551-562.
Tsai, C. S., Huang, Y. S., Wu, C. L., Hwang, F. M., Young, K. B., Tsai, M. H., & Chu, S. M. (2013). Long-term effects of stimulants on neurocognitive performance of Taiwanese children with attention-deficit/hyperactivity disorder. BMC psychiatry, 13(1), 330.

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IQ in ADHD sufferers indeed increases with the treatment of stimulants, such as methylphenidate (Meyes et al., 1994). This is thought to rely on the attention-narrowing effect of stimulants, i.e., they cause the easily distracted ADHD child to focus better on the IQ task (Aman, 1996).

It is an interesting question indeed whether such an effect also plays a role in healthy controls. Indeed it does, at least according to one study, where a control population was subjected to amphetamine.

  • Amphetamine improves performance on spatial working memory, Stroop tasks and enhances language production (Barch & Carter, 2005). The authors did, however, not measure performance on full blown IQ tests.

References
- Aman, J Development Phys Disabil (1996); 8(4): 347-65
- Barch & Karter, Schizophrenia Res (2005); 77(1): 43-58
- Meyes et al., DMCN (1994); 36(12): 1099–107

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    $\begingroup$ Nice find. It didn't occur to me they would give amphetamine to schizophrenics (or their control counterparts). $\endgroup$ – Fizz Dec 22 '17 at 9:22
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Not about IQ as a whole alas, but I found a 2010 systematic review & meta-analysis "Modafinil and methylphenidate for neuroenhancement in healthy individuals: A systematic review", apparently the only systematic one to date on methylphenidate in healthy subjects:

The term neuroenhancement refers to improvement in the cognitive, emotional and motivational functions of healthy individuals through, inter alia, the use of drugs. Of known interventions, psychopharmacology provides readily available options, such as methylphenidate and modafinil. Both drugs are presumed to be in widespread use as cognitive enhancers for non-medical reasons. Based on a systematic review and meta-analysis we show that expectations regarding the effectiveness of these drugs exceed their actual effects, as has been demonstrated in single- or double-blind randomised controlled trials. Only studies with sufficient extractable data were included in the statistical analyses. For methylphenidate an improvement of memory was found, but no consistent evidence for other enhancing effects was uncovered. Modafinil on the other hand, was found to improve attention for well-rested individuals, while maintaining wakefulness, memory and executive functions to a significantly higher degree in sleep deprived individuals than did a placebo. However, repeated doses of modafinil were unable to prevent deterioration of cognitive performance over a longer period of sleep deprivation though maintaining wakefulness and possibly even inducing overconfidence in a person's own cognitive performance.

Somewhat similarly, there's another more recent (2014) review of methylphenidate "Cognitive effects of methylphenidate in healthy volunteers: a review of single dose studies" (MPH), but its methodology is limited to "effective study percentage" (i.e. not fully meta-analytical):

The studies reviewed here show that single doses of MPH improve cognitive performance in the healthy population in the domains of working memory (65% of included studies) and speed of processing (48%), and to a lesser extent may also improve verbal learning and memory (31%), attention and vigilance (29%) and reasoning and problem solving (18%), but does not have an effect on visual learning and memory. MPH effects are dose-dependent and the dose–response relationship differs between cognitive domains. MPH use is associated with side effects and other adverse consequences, such as potential abuse. Future studies should focus on MPH specifically to adequately asses its benefits in relation to the risks specific to this drug.

If one is willing to consider modafinil in this class of drugs (some have labelled it "atypical DAT-inhibitor"), then there's a newer 2015 review "Modafinil for cognitive neuroenhancement in healthy non-sleep-deprived subjects: A systematic review":

We found that whilst most studies employing basic testing paradigms show that modafinil intake enhances executive function, only half show improvements in attention and learning and memory, and a few even report impairments in divergent creative thinking. In contrast, when more complex assessments are used, modafinil appears to consistently engender enhancement of attention, executive functions, and learning.

All reviews primarily point to memory effects (in the short term, laboratory tests), although the last review (modafinil-only) has more mixed results in that regard. The other potential effects of MPH on IQ like speed of processing, are less convincing.

Although I was asking about IQ here, the long-term effects analyzed in the second review raise interesting questions about the mediating mechanism for the broader enhancement effects like learning (these "neuroenhancers" being popular as "study drugs"). There's a different line of research indicating that stimulants may affect motivation even more than mere cognition, even for "healthy" individuals, although apparently only in some sub-groups... which raises other interesting questions (sub-clinical attention problems, personality factors, etc.)

Actually, the authors is this last line of research have conducted their own (2015) meta-analysis of MPH and amphetamine: "Prescription Stimulants' Effects on Healthy Inhibitory Control, Working Memory, and Episodic Memory: A Meta-analysis.":

The present meta-analysis was conducted to estimate the magnitude of the effects of methylphenidate and amphetamine on cognitive functions central to academic and occupational functioning, including inhibitory control, working memory, short-term episodic memory, and delayed episodic memory. In addition, we examined the evidence for publication bias. Forty-eight studies (total of 1,409 participants) were included in the analyses. We found evidence for small but significant stimulant enhancement effects on inhibitory control and short-term episodic memory. Small effects on working memory reached significance, based on one of our two analytical approaches. Effects on delayed episodic memory were medium in size. However, because the effects on long-term and working memory were qualified by evidence for publication bias, we conclude that the effect of amphetamine and methylphenidate on the examined facets of healthy cognition is probably modest overall. In some situations, a small advantage may be valuable, although it is also possible that healthy users resort to stimulants to enhance their energy and motivation more than their cognition.

Its results are mostly in line with previous conclusions (memory being the main aspect improved), however it emphasized that effect size is small (for short-term effects), and it also found publication bias for long-term effects.

After writing that, I found another 2015 review which mostly summarizes what I said by going over those meta-analyses, but emphasizes memory effects and doesn't dig much the motivation explanation.

Finally, I found a 2017 RCT on chess performance (which is reasonably correlated with IQ, at least for non-top players) and psychostimulants. The results of this study are not straightforward:

This study investigated whether performance in chess – a competitive mind game requiring highly complex cognitive skills – can be enhanced by methylphenidate, modafinil or caffeine. In a phase IV, randomized, double-blind, placebo-controlled trial, 39 male chess players received 2×200 mg modafinil, 2×20 mg methylphenidate, and 2×200 mg caffeine or placebo in a 4×4 crossover design. They played twenty 15-minute games during two sessions against a chess program (Fritz 12; adapted to players’ strength) and completed several neuropsychological tests. Marked substance effects were observed since all three substances significantly increased average reflection time per game compared to placebo resulting in a significantly increased number of games lost on time with all three treatments. Treatment effects on chess performance were not seen if all games (n=3059) were analysed. Only when controlling for game duration as well as when excluding those games lost on time, both modafinil and methylphenidate enhanced chess performance as demonstrated by significantly higher scores in the remaining 2876 games compared to placebo. In conjunction with results from neuropsychological testing we conclude that modifying effects of stimulants on complex cognitive tasks may in particular result from more reflective decision making processes. When not under time pressure, such effects may result in enhanced performance. Yet, under time constraints more reflective decision making may not improve or even have detrimental effects on complex task performance.

Quite an interesting conclusion: to me it suggests that modafinil and MPH enhance exploitation rather than exploration.

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  • $\begingroup$ TY for the post as i'm preparing a paper on CE drugs right now. can you clarify what you mean by "exploitation rather than exploration" $\endgroup$ – faustus Jul 19 '18 at 11:53
  • $\begingroup$ @faustus: under MPH/modafinil more games were lost on time; I interpret that as as the doped players overanalyzing the game tree in a losing section; sometimes that's called "Analysis paralysis". If however the doped players happened to "overanalyze" in a profitable area of the game tree, then they got some payoff (in terms of slightly better moves) for their deeper analysis. It seems in line with the (rather scant) previous reports that modafinil resulted in some "impairments in divergent creative thinking". $\endgroup$ – Fizz Jul 19 '18 at 16:53
  • $\begingroup$ @faustus: or to put it in game tree terms; it looks like doping with MPH/modafinil gives more depth at the expense of breadth of evaluation. $\endgroup$ – Fizz Jul 19 '18 at 17:39

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