Participants and procedure
We conducted a cross-sectional representative survey of the German population between August 19th and September 19th 2016 regarding attitudes, perceptions and behaviours referring to PNE. Face-to-face interviews of approximately 30 min length were conducted in 1128 people at participant’s place (minimum age: 18 years). The individuals were selected by the “Institut für Demoskopie Allensbach” as they met criteria of the quota sample based on the German official statistics regarding central socio-demographic factors . This procedure was chosen to increase the generalizability of the results for the German population. Participants were informed by professional interviewers about the objectives of the study, the procedure of data storage and confirmed their voluntary participation verbally. To help to assure confidentiality, there was no written consent. To ensure that each question is comprehensible, the standardized questionnaire was pretested and optimised.
The interviewers were trained uniformly to answer further questions if necessary for example if there were uncertainties in the understanding of terms such as “better cognitive performance”. Hence they especially clarified that the questions regarding PNE do not concern the intake of medication by doctor’s prescription. The questionnaire included a section assessing socio-demographic data such as gender, age, school education, current or last professional position, employment status, working hours, shift work, federal state of residence, size place of residence and soft enhancer intake.
Also there were questions regarding the perception of the topic PNE in the media, ethical questions and socio-psychological aspects which will be published separately.
Beside questions that were directly asked by the interviewer, the questionnaire also included a section that had to be completed written and returned hidden in an envelope to the interviewer to increase the reliability of the prevalence rates of PNE use. As these questions referred to the individual consumption of psychoactive substances to increase individual performance, this procedure guaranteed confidentiality for the respondent regarding these sensitive questions. The study was approved by the local Ethics Committee (Landesärztekammer Rheinland-Pfalz, No 837.209.14, 9448F), and there was no remuneration of the participating subjects.
Assessment of prevalence of PNE and reasons and goals for intake
The questionnaire to assess the prevalence of PNE as well as goals and attitudes towards PNE by our group was based on experiences with earlier surveys [4, 16] and other published data.
Assessment of prevalence of PNE
To estimate the prevalence of PNE, we asked two main questions: The first question referred to enhancement by the intake of freely available substances that can be bought in supermarkets or pharmacies without any prescription such as energy drinks, Ginkgo biloba or caffeine tablets. The interviewer asked: “There are various substances mentioned on this list. Are there any that you have taken or are currently taking to improve your mental performance, improve your mood, relieve anxiety or nervousness, or manage stress? You only have to tell me the corresponding numbers from the list.” The second question referred to the use of prescription and/or illicit drugs. It was assessed within the self-completed anonymous paper-and pencil-section. Out of 1128 respondents, 86 had never heard of the phenomenon of PNE. Thus they were assigned to the group of “non-users” in the further analysis. Hence 1042 had been asked for their use of prescription/illicit substances for PNE.
Here, the interviewer asked: “Here are some drugs that can be used to improve cognitive performance, improve mood, relieve anxiety or nervousness, or to manage stress. Please tick every drug if you have already taken it for the above-mentioned purpose without medical indication. If you have already taken a drug, please indicate when and how often.” The list of substances included prescription drugs, such as methylphenidate (e.g., Ritalin®) or modafinil, and illicit substances such as cocaine or amphetamines. All substances were clustered into three substance groups (stimulants, mood enhancer and cannabis). For a more differentiated consideration “stimulants” were divided into “prescription stimulants” and “illicit stimulants” for further analysis. If one substance of a substance group was used at least once for PNE, the respondent was asked to answer further questions about the use. If more than one substance of a substance group was used, it was asked to choose the most important substance of that group and the respondent had to answer further questions about the use only for that substance of the group.
Assessment of reasons and goals for PNE
Beside the frequency of intake, participants were also asked for the reason of their use and which goals they pursued by the intake of the substance. The two questions were asked for one substance of each substance group. If they used multiple substances of one group (stimulants, mood enhancer, cannabis) they were asked to indicate reasons and goals only for the use of the most important one. They were asked: “For what reason did you take this drug? Please tick for every reason stated to what extend this applies to you.” and “What were you trying to achieve by taking this drug? Please tick for every reason stated to what extend this applies to you.” Users could rate the importance of different reasons and goals for the intake of the substance on a five-point Likert scale (1 = I totally agree; 5 = I totally disagree) e.g., “I wanted to improve mood” or” I wanted to be able to handle stressful situations better”. The coding of items was reversed (1 = I totally disagree; 5 = I totally agree) before calculating mean values thus higher values mean higher importance of the reason or goal for the substance use. As individuals might be part of different substance groups there are no p-values to estimate statistical significance of mean values.
Questionnaires to assess the ability to recover from stress, perceived stress and related psychological resilience factors
To allow for analyses of the association between PNE and the ability to cope with stress, different assessment scales were used:
Brief Resilience Scale (BRS) 
The scale consists of six items assessing self-ratings of the individual ability to recover from stress despite significant adversity (item 1” I tend to bounce back quickly after hard times”, item 2” I have a hard time making it through stressful events”, item 3” It does not take me long to recover from a stressful event”, item 4” It is hard for me to snap back when something bad happens”, item 5” I usually come through difficult times with little trouble” and item 6” I tend to take a long time to get over set-backs in my life”). The items are rated on a five-point Likert scale (1 = strongly disagree; 5 = strongly agree). Item 1, 3, and 5 are positively phrased; items 2, 4, and 6 are negatively phrased. The coding of the negatively phrased items is reversed in order to calculate the mean (range: 1–5) of the six items . Higher values indicate a higher ability to recover from stress. We used the German version of the instrument, which was recently validated in a population of n = 2.609 German participants . The psychometric data of the BRS ratings of the sample investigated here were part of this validation study.
Perceived Stress Scale (PSS-4) 
The PSS-4 consists of four items measuring the individual evaluation of stressful situations in the previous 12 months (item 1 “How often have you felt that you were unable to control the important things in your life?”, item 2.
“How often have you felt confident about your ability to handle your personal problems?”, item 3 “How often have you felt that things were going your way?”, item 4 “How often have you felt difficulties were piling up so high that you could not overcome them?”). The items are rated on a five-point Likert scale (1 = never; 5 = very often). Item 2 and 3 are reverse coded and were recoded for the analysis. For each subject, sum scores across all items are calculated (range: 0–16). Higher values indicate more perceived stress. We used the German version of the scale .
Short Scale for Measuring General Self-efficacy Beliefs (ASKU) 
The questionnaire consists of three positively worded items assessing self-rated confidence in the individual ability to achieve intended results (“I can rely on my own abilities in difficult situations”, “I am able to solve most problems on my own”, “I can usually solve even challenging and complex tasks well”) rated on a five-point Likert scale (1 = does not apply at all; 5 = applies completely). Mean scores are used for analysis (range: 1–5). Higher values indicate higher self-efficacy.
Short Scale for the Assessment of Locus of Control (IE) 
The four-item scale assesses internal and external control beliefs (internal control beliefs: “I’m my own boss”, “If I work hard, I will succeed”; external control beliefs: “Whether at work or in my private life: What I do is mainly determined by others”, “Fate often gets in the way of my plans”). The questions are rated on a five-point Likert scale (1 = does not apply at all; 5 = applies completely). Mean scores for internal or external control are calculated (range: 1–5). Higher values indicate higher internal (items 1 and 2) or external (items 3 and 4) control beliefs.
Optimism-Pessimism-2 Scale (SOP-2) 
The questionnaire consists of two items assessing self-rated optimism (“How optimistic are you in general?”) and pessimism (“How pessimistic are you in general?”). The questionnaire uses a seven-point Likert scale (optimism: 1 = not at all optimistic; 7 = very optimistic; pessimism: 1 = not at all pessimistic; 7 = very pessimistic). To calculate the mean of the two items, reverse scoring of the item pessimism is used (range: 1–7).
Analyses were performed with SPSS for Windows, Version 17.0. To guarantee representativeness to the highest possible standard, data of respondents included in the final sample was weighted for the area and federal states of Germany, size of the town, gender, school education, age and profession. N values of weighted data were rounded. Differences between users or subgroups of users and non-users in sociodemographic variables (gender, age, education, current or last professional position, employment status, working hours, shift work, federal state of residence, size place of residence and soft enhancer intake) were analysed using chi2-test, Fisher’s exact test, t-test and Welch test. Variables referring to the importance of reasons and goals were recoded for analysis (1 = I totally disagree; 5 = I totally agree). Means are reported with standard deviations (SD).
In order to assess the associations between PNE use and the ability to recover from stress, perceived stress and resilience factors (self-efficacy, control beliefs and optimism), stepwise logistic regression with backward elimination was conducted to determine predictors of PNE consumption using the most parsimonious model. Prior to multivariate analyses, means of users (and respective subgroups) with nonusers were compared using t-tests for continuous variables to assess the associations between each of the potential predictors (BRS, PSS-4, ASKU, IE internal, IE external, SOP-2) with PNE use. To test for multicollinearity, associations between the predictor variables (Pearson correlations) were examined and the variance inflation index (VIF) calculated. According to the literature, the correlations should not exceed .80, the VIF should not exceed 10 .
In the multivariate model, all predictors (BRS, PSS-4, ASKU, IE internal, IE external, SOP-2) were excluded stepwise. In order to control for potential confounders, sociodemographic variables (gender, age, education, current or last professional position, employment status, working hours, shift work, federal state of residence, size place of residence, soft enhancer intake) with a statistically significant mean difference between users or subgroups of users and non-users (p < .1) were included at once (method: enter) from the first step on in the model. Continuous variables (age, BRS, PSS-4, IE external, IE internal, SOP-2 and ASKU) were z-transformed. The significance level was p < 0.05.