Anecdotal evidence suggests that cannabis intoxication enhances human creativity. In line with that, Steve Jobs, an undeniably creative mind, once stated: “The best way I could describe the effect of the marijuana and hashish is that it would make me relaxed and creative.” Other regular users claim that cannabis induces a state in which they experience unusual and original thoughts (Tart 1970). In a more recent review, over 50 % of users reported heightened creativity during cannabis intoxication (Green et al. 2003). This widespread perception of cannabis as a creativity-enhancer makes it important to verify whether cannabis actually induces these supposed effects. Delta-9-tetrahydrocannabinol (THC), the main psychoactive compound present in the Cannabis sativa plant, has been found to reduce inhibitory control (McDonald et al. 2003) and stimulate striatal dopamine (DA) release (Bossong et al. 2009; Kuepper et al. 2013). These features of THC intoxication, in turn, are expected to play a role in particular aspects of creative thinking (Akbari Chermahini and Hommel 2010; Hommel 2012). On the other hand, THC has been linked to the emergence of psychotic symptoms both due to acute administration (D’Souza et al. 2004), as well as in the long-term (Kuepper et al. 2010). As a result, the possible beneficial effects of using cannabis, if any, might not outweigh the potential risks associated with its abuse.

The concept of creativity is not very well defined, and there is no agreement on one particular measure how to assess it. While some authors consider the concept to refer to the product of creative activities, others take it to reflect the personality of the product’s creator (for an overview, see Runco 2007). To circumvent these difficulties, we restricted our analyses to two well-established creative processes and the respective classical assessment methods: divergent and convergent thinking (Guilford 1967). Divergent thinking takes place when people try to find as many solutions to a loosely defined problem as possible—a process often referred to as “brainstorming.” It is often assessed by means of Guilford’s (1967) Alternate Uses Task (AUT), which requires individuals to generate as many as possible uses for a common household item (such as a pen or book) as they can think of (e.g., reading it, using it as a doorstop, etc.). In contrast, convergent thinking takes place when trying to find the one possible solution to a very well-defined problem. This process is often assessed by means of Mednick’s (1962) Remote Associates Task (RAT), in which people are presented with three supposedly unrelated concepts (e.g., “time,” “hair,” “stretch”) and are requested to identify the one concept that can be related to all three of them (“long”). Research indicates that performance in AUT and RAT is not (strongly) correlated (Akbari Chermahini and Hommel 2010; Akbari Chermahini et al. 2012). Moreover, there is evidence that the two types of creative thinking are differently related to subcortical DA levels: While divergent thinking performance relates to markers of DA levels in the form of an inverted U-shape, convergent thinking performance displays a linear, negative correlation with DA markers (Akbari Chermahini and Hommel 2010). In addition, this dissociation of human creativity seems to correspond to the dual pathway to creativity model (De Dreu et al. 2008; Nijstad et al. 2010) suggesting that creative performance emerges from the balance between cognitive flexibility and cognitive persistence—two dissociable cognitive control functions (De Dreu et al. 2012).

With regard to the neural effects of THC, the link between creative thinking and DA appears to be particularly interesting. Administration of THC has been shown to indirectly induce DA release in the striatum (Bossong et al. 2009; Kuepper et al. 2013), and there is evidence that its chronic application can lead to dopaminergic hypoactivity in the long-term, especially if the onset of cannabis use is at a young age (Hoffman et al. 2003; Urban et al. 2012; Bloomfield et al. 2014). As divergent thinking performance is expected to be optimal with medium subcortical DA levels (Akbari Chermahini and Hommel 2010), one may suspect that THC can have a beneficial effect on this creative process, particularly in individuals with low dopaminergic functioning. This assumption is further supported by the fact that the reduction in inhibitory control, as observed in response to stimulation by pure THC (McDonald et al. 2003) and cannabis (Ramaekers et al. 2006; Ramaekers et al. 2009), has been related to dopaminergic functioning as well (Mink 1996). Reduced inhibitory control can be considered to reflect a cognitive control state with weak top-down guidance. Such a state should affect convergent and divergent thinking differently (Hommel 2012). As pointed out by Bogacz (2007), human decision-making and the retrieval of possible alternatives can be considered a process that emerges from the interaction of top-down guidance and low-level competition between alternatives. If so, convergent thinking, with its many top-down constraints targeting one single solution, would seem to require a control state that provides strong top-down guidance and strong local competition. In contrast, divergent thinking, with its loosely defined problem and its many solutions, seems to require a control state that provides weak top-down guidance and only little local competition (Hommel 2012). To the degree that THC indeed induces a control state with weak top-down guidance and local competition, it might thus be expected to improve divergent thinking, interfere with convergent thinking, or both (Hommel 2012; Colzato et al. 2012).

Unfortunately, the available research on the link between cannabis and creativity allows only for partial verification of these expectations. With respect to divergent thinking, one study showed that subjects intoxicated with joints (cannabis cigarettes) containing a low dose of THC (3 mg in total) displayed significantly enhanced performance on two divergent production tasks, compared to a group that received a higher THC dose (6 mg in total; Weckowicz et al. 1975). Curran et al. (2002) showed that, as compared to placebo, oral THC (7.5 and 15 mg) dose-dependently improved verbal fluency—an important aspect of divergent thinking (Guilford 1967), at least as assessed by the AUT. Improved verbal fluency performance was also found in a naturalistic study that showed beneficial effect of smoked cannabis (10 % THC on average) on divergent thinking to be restricted to users low in trait creativity (i.e., individuals that obtained a low score on a self-assessment questionnaire about achievements in different creative domains; Schafer et al. 2012). In addition to fluency, cannabis administration (joints containing 19 mg of THC) has also been shown to increase the number of original responses on a test of associative processes, in comparison to placebo (Block et al. 1992). In contrast, Tinklenberg et al. (1978) did not observe any improvement in performance during the Torrance Tests of Creative Thinking (TTCT; Torrance 1966), which is often assumed to tap into divergent thinking, after oral consumption of THC (a biscuit containing 0.3 mg/kg body weight of THC). Another study found decreased TTCT scores for fluency, flexibility, and elaboration after smoking a cannabis joint (containing 10 mg of THC) in regular cannabis users but not in first-time users (Bourassa and Vaugeois 2001). In summary, the methodological differences between the various studies aside, many but not all findings suggest that THC may induce a cognitive control state with weak top-down guidance, thus efficiently decreasing the competition between cognitive representations and enhancing divergent thinking (Hommel 2012; Colzato et al. 2012).

For convergent thinking, the evidence is even more limited. Weckowicz et al. (1975) observed a trend toward less efficient convergent thinking task after smoking joints containing a low dose of THC (3 mg in total) or a higher dose (6 mg in total), in comparison to both a placebo and a pure control group. However, the same study also found impaired convergent thinking but only for the high-dose condition. The most recent investigation found potentially detrimental effects of smoking cannabis (10 % THC on average) on RAT performance in a group of cannabis users assumed to be high in trait creativity (Schafer et al. 2012). Although the naturalistic approach of this study makes it difficult to account for specific dose-related differences, the results of the research of both Schafer et al. (2012) and Weckowicz et al. (1975) suggest that THC can disrupt the process of searching and converging on a single solution to a problem.

A number of the observed inconsistencies between studies might be due to differences with respect to THC dosage and method of administration, which, in turn, affects the bioavailability and the onset of action of the compound (Hazekamp et al. 2006). Moreover, an individual’s history of cannabis use needs to be identified before cognitive changes in response to THC can be predicted. Administration of joints (containing up to 39 mg of THC) to regular cannabis users has been found to produce no accuracy impairments on a test battery assessing several cognitive functions (Hart et al. 2001) and, more specifically, on tasks related to episodic and working memory (Hart et al. 2010). Furthermore, after smoking a cannabis joint (containing 500 μg/kg body weight THC), chronic users did not display any behavioral deficiencies on tasks assessing tracking performance and divided attention (Ramaekers et al. 2009) or changes in an event-related potential (ERP) reflecting early attentional processes (Theunissen et al. 2012), compared to infrequent users. In addition, regular cannabis users were shown to display reduced sensitivity to the psychotomimetic effects of THC (administered as an intravenous dose up to 5 mg; D’Souza et al. 2008). In contrast, inhibitory control has been found to be similarly impaired among both occasional and chronic users when intoxicated with cannabis (Ramaekers et al. 2009).

Accordingly, since research points to reduced cannabinoid receptor type 1 (CB 1 ) density in the brains of regular cannabis users (Hirvonen et al. 2012), one may suspect that the tolerance of chronic users to some of the detrimental effects of THC is, to some extent, related to their dopaminergic functioning. Specifically, due to the concentration of CB 1 receptors at gamma-aminobutyric acid (GABA) and glutamate neurons, CB 1 receptor downregulation can influence the activity of these neurotransmitters (Hoffman et al. 2003). Because DA neurons are frequently co-localized with GABAergic and glutamatergic terminals, the dopaminergic deficiencies observed in chronic cannabis users may be explained by lasting, maladaptive modulation of DA by GABA and glutamate (Fattore et al. 2010; Fernández-Ruiz et al. 2010). If so, keeping in mind the inverted U-shape relationship between subcortical DA levels and divergent thinking performance (Akbari Chermahini and Hommel 2010) and the effect of THC on striatal DA release (Bossong et al. 2009; Kuepper et al. 2013), it may be expected that individuals with a relatively low level of dopaminergic functioning, such as regular cannabis users, are more likely to demonstrate enhanced performance on a divergent thinking task, provided that the THC dose is not excessively high. In contrast, in a population without long-term dopaminergic imbalances, such as healthy drug-naïve individuals, even a reasonably low dose of THC could stimulate DA production to the level that it exceeds the threshold for optimal performance. In the case of convergent thinking performance, which is best with low subcortical DA levels (Akbari Chermahini and Hommel 2010), it may be predicted that it will deteriorate in response to THC, irrespective of the dose and cannabis use history of the individual.

In order to examine these possibilities, we investigated the effect of two different doses of vaporized cannabis (containing 5.5 or 22 mg THC; see “Study drugs” section) and placebo on convergent and divergent thinking in a sample of chronic cannabis users, using a between-groups design. On the basis of the assumption that a low dose of cannabis can remove potential impairments caused by regular use (Weckowicz et al. 1975; Kelleher et al. 2004), we expected that participants intoxicated with a low dose of cannabis should display higher scores on a divergent thinking task, compared to placebo. Conversely, we predicted impairment of performance in the high-dose condition, in contrast to the low-dose and placebo conditions. In the case of convergent thinking, we expected that both doses of cannabis should impair this process, compared to placebo. In addition, since divergent thinking performance has been found to be related to an individual’s mood (Zenasni and Lubart 2011), we assessed perceived mood as a possible modulating factor.