Effects of recreational marijuana legalization on college students: A longitudinal study of attitudes, intentions, and use behaviors

Abstract

Purpose:

As legal recreational marijuana use expands rapidly across the United States, there is growing concern that this will lead to higher rates of use among college-aged young adults. Given the limited research addressing this issue, a longitudinal study was conducted to evaluate the effects of legalizing recreational use on the attitudes, intentions, and marijuana use behaviors of college students in two different legalization contexts, Washington State and Wisconsin.

Methods:

Survey data assessing marijuana attitudes, intentions, and use behavior were collected from 2011 to 2016 on a longitudinal cohort of 338 students at two large public universities in Washington and Wisconsin. Time series analyses were conducted to evaluate post-legalization changes in ever use, 28-day use, and mean attitude and intention-to-use scores in Washington state, using Wisconsin participants as the control group.

Results:

Ever use, attitude, and intention-to-use scores did not change significantly more in Washington after legalization than in Wisconsin. However, among prior users, the proportion using in the last 28 days rose faster in Washington after legalization that it did in Wisconsin (p<0.001).

Conclusions:

Findings suggest that legalization had the greatest effects on current marijuana users, who are surrounded by a climate that is increasingly supportive of its use.

Implications and contribution:

This longitudinal study evaluated college students from two states, one with legal recreational marijuana and one without. Among prior users, current use increased in the state with legal recreational after age 21 compared to the state without. Findings suggest that legal recreational use may facilitate chronic use among college students.

INTRODUCTION

Marijuana is among the drugs most commonly used by young adults, with over 50% of college students reporting prior use and 21.3% using in the past 30 days [1]. Despite young adults perceiving marijuana more positively than cigarettes, electronic cigarettes, hookah, and cigars [2], it is associated with increased cognitive, mental health, and substance use disorders, including difficulties with verbal performance, attention, working memory, and inhibition [3]. Many of these health concerns fall disproportionately on young users, due to detrimental effects of the drug on the prefrontal cortex, which continues to develop into the mid-20s [4, 5].

As of 2019, it is legal for adults aged 21 and older to use marijuana for recreational purposes in 10 states and medical purposes in an additional 23. While medical marijuana has been legal in California since 1996, recreational marijuana legalization (RML) first passed in only November 2012, in the states of Washington and Colorado. Washington’s Initiative 502 and Colorado’s Amendment 64 legalized possession of up to one ounce of marijuana for recreational use among adults 21 and older starting in December 2012. Dispensaries allowing commercial sale in each state subsequently opened in July 2014 and January 2014, respectively.

There is growing concern that the wider access provided by RML will lead to increased use of marijuana among young adults, including those college-aged students for whom the practice is still illegal. Most studies to date have focused on the effects of legalizing medical marijuana [6], since legal recreational use is a much more recent development. However, RML provides greater access to marijuana than legalization for medical purposes and the results from the initial medical marijuana literature are not generalizable to the current legal context.

The existing studies that evaluate the impact of RML on the use practices of college students have had conflicting results. Miller et al.’s study found that after Washington legalized recreational marijuana in 2012, rates of marijuana use in the last 30-days increased among students at Washington State University more than the national average [7]. Jones’s et al. focused on the 2014 opening of dispensaries as the primary intervention timepoint and found that in Colorado, there was no significant difference in use frequency throughout the 2013 to 2015 study period [8].

RML passed in Oregon in November 2014 and took effect in July 2015. Kerr et al.’s 2017 study showed a significant increase in the rate of 30-day marijuana use from 2012 to 2016 across most participating colleges, including the Oregon school. However, the post RML change in 30-day use was only significantly greater in Oregon among students with recent heavy alcohol use [9]. Yet, a second Oregon study found that RML was significantly associated with increased 30-day marijuana use for all participants, even after controlling for perceived peer use [10].

All four studies are limited by utilizing cross-sectional surveys, rather than longitudinal data collected on a cohort of students over time. Additional work is also needed to characterize these effects, as the studies focused on marijuana use behavior, without collecting critical data regarding students’ attitudes towards marijuana. A longitudinal study was already collecting data on college substance use in Washington and Wisconsin prior to Washington’s legalization of recreational marijuana possession in 2012. Recreational marijuana continues to be illegal in the state of Wisconsin, to date. Thus, RML in Washington provided a natural experiment to evaluate the effects of legalizing marijuana possession on college students in two states with different legalization contexts. Using these data we conducted a longitudinal study of the effects of RML on the attitudes and behaviors of college students in Washington and Wisconsin.

METHODS

Approach

This study relies on interview data collected from May 2011 to September 2016 on a cohort of 338 students at two large public universities in Washington and Wisconsin (Table 1). It takes place in the context of an overarching mixed methods study of substance use and social media practices that was conducted over the same time period [11, 12].

Table 1:

Demographic information

Characteristic	Summer pre-freshman year (survey cycle 1, 2011)			Summer one year post-graduation (survey cycle 6, 2016)
	WI n = 199, (%)	WA n = 139, (%)	p-value	WI n = 101, (%)	WA n = 54, (%)	p-value
Gender: Female	111 (55.8)	79 (56.8)	0.94	60 (59.4)	34 (63.0)	0.80
Ethnicity: White	177 (88.9)	76 (54.7)	<0.001	90 (89.1)	29 (53.7)	<0.001
Asian	8 (4.0)	31 (22.3)		6(5.9)	13 (24.1)
Other1	14 (7.0)	32 (23.0)		5 (5.0)	12 (22.2)
Housing: On campus (versus off campus)	189 (95.0)	88 (63.8)	<0.001	-	-	-
Proportion of friends who approve marijuana use: mean %, (SD)	37.4 (30.1)	43.2 (31.4)	0.09	57.6 (25.9)	60.4 (23.7)	0.50
Proportion of friends who used marijuana in the last 28 days: mean %, (SD)	27.1 (27.3)	32.5 (29.3)	0.09	32.2 (24.5)	38.1 (21.3)	0.13

¹Other includes participants who self-identify as Hispanic, African American, Native American, Alaskan, East Indian, multiple ethnicities, or other

In November 2012, after two cycles of data collection, the state of Washington passed Initiative 502. With this law, possession of up to one ounce of marijuana became legal for adults 21 and older, effective December 6, 2012. Sale of marijuana became legal when dispensaries first opened in July 2014. Throughout the study period, recreational marijuana possession, cultivation, and sale remained illegal in Wisconsin. Currently, only medical cannabidiol oil is allowed in the state. This provided a natural experiment to assess the effects of RML on participants in Washington, using Wisconsin participants as a control group. The pre-intervention period spanned from May 2011 to September 2012 and post-intervention period spanned from May 2013 to September 2016, corresponding to the effective intervention date of December 6, 2012.

Participants and recruitment

Participants were randomly selected for inclusion in the cohort from 2011 registrars’ lists of incoming first year students at the two institutions. Eligibility criteria included age 17–19 years, English speaking, and pending full-time enrollment. Students were recruited to the study during the summer before their freshman year and interviewed annually for five cycles (Figure 1). A sixth interview cycle was added for a subset of participants, randomly selected for continued participation from within each university cohort. A 30- to 60-minute phone interview was conducted each summer by trained research assistants. The study received approval from the relevant Institutional Review Boards at both universities.

Figure 1:

Interview participants, May 2011 – September 2016

In order to promote honesty among participants, at the beginning of each interview they were asked “Are you in a location where you feel comfortable and that your privacy will be protected?” This was followed by the reminder “All of the information you give us in this study is additionally protected by a certificate of confidentiality, which means we may and will protect your identity and information from compelled disclosures, even if made by a court of law.”

Measures

Data regarding participants’ demographics and marijuana related attitudes, intentions, and behaviors were collected at all six interview timepoints, based on self-reported responses. In addition to gender, ethnicity, and housing status, information was also collected regarding the approval and use practices of participants’ friends: “What percentage of your friends approve of the use of marijuana?” and “What percentage of your friends use marijuana currently, in the last 28 days?” respectively.

Study questions were developed based on the Theory of Planned Behavior. This conceptual framework has been widely used across many fields, including the adolescent substance use literature [13], and suggests that one’s attitudes towards a given behavior affect their intentions to conduct the behavior. These intentions, in turn, drive one’s actual behaviors. Attitude towards marijuana was measured using a 7-point Likert scale (higher scorer=more positive attitude): “This question is about your attitude towards marijuana in general. On a scale between 0 and 6, with 0 as ‘very negative,’ 3 as ‘neutral,’ and 6 as ‘very positive,’ what would you say your own attitude towards marijuana is?” Intention to use marijuana in the next six months was measured using a 6-point Likert scale (higher score=higher intention): “How likely do you think it is that you will use marijuana in the next 6 months? Please answer from 0 ‘not at all likely’ to 5 ‘very likely.’” [12]. Marijuana use behavior was evaluated by assessing for any prior use: “Have you ever used marijuana in your life?” Among prior users, we also evaluated for current use, defined as any use in the past 28 days: “Have you had any marijuana products in the last 28 days? This is just under a month and would include 4 weekends. Please feel free to take your time and look over your calendar.”.

Analyses

Pairwise comparisons were preformed using chi-squared, Fisher’s exact, and t tests, as appropriate. Time series analyses were conducted using Prais-Winsten regression with robust variance estimation, to account for first-order autocorrelation between measurements. Statistical analyses were performed in Stata version 14.0 (StataCorp: College Station, TX) using an alpha significance level of 0.05.

RESULTS

A total of 338 participants initially enrolled in the study. They were 56% female, 75% white, and 41% from Washington State (Table 1). These demographics remained similar throughout the study, with the subset of participants remaining in the final surveyed population composed of 60% female participants, 77% white, and 35% from Washington State. There was no difference between states regarding the mean proportion of participants’ friends who approved or used marijuana at either time period. Baseline users were slightly less likely than never users at baseline to drop out of the study by interview cycle 6, although this difference was non-significant (52.2% vs 55.2% attrition, respectively, p=0.60).

In any given interview year, there was no significant difference between Washington state and Wisconsin regarding the proportion of participants who reported ever using marijuana (Table 2). This proportion rose steadily from 42.9% and 33.2% in the initial survey the summer prior to starting freshman year (Washington and Wisconsin, respectively; p=0.78), to 64.8% and 72.2% (Washington and Wisconsin; p=0.44) at the final survey one year after graduation. Time series analysis demonstrated no difference between the two states with regards to the rate at which the incidence of ever-using marijuana changed after, compared to before, RML (Table 3).

Table 2:

Attitude, Intention, and Behavior Related Outcomes

	Pre-intervention							Post-intervention													p-value
	2011			2012			Pre	2013			2014			2015			2016			Post
	WI	WA	p-value	WI	WA	p-value		WI	WA	p-value	WI	WA	p-value	WI	WA	p-value	WI	WA	p-value
Sample size	199	139		187	128			162	121		155	110		152	108		101	54
Attitude: mean, (SD)	2.0 (1.7)	1.8 (1.7)	0.48	2.4 (1.9)	2.2 (1.7)	0.34	2.1	2.7 (1.7)	2.6 (1.7)	0.56	2.8 (1.6)	2.7 (1.4)	0.65	3.0 (1.7)	3.0 (1.5)	0.90	3.1 (1.6)	2.9 (1.6)	0.57	2.8	<0.001
Intention: Mean, (SD)	0.9 (1.4)	0.4 (0.9)	<0.001	1.2 (1.7)	1.1 (1.7)	0.60	0.9	2.0 (2.1)	1.7 (2.0)	0.13	1.8 (2.0)	1.8 (2.0)	0.60	1.6 (1.9)	1.7 (2.0)	0.42	2.0 (2.1)	1.8 (2.1)	0.66	1.8	<0.001
Ever used: n, (%)	66 (33.2)	49 (35.2)	0.78	89 (47.6)	59 (46.1)	0.88	-	93 (57.4)	62 (51.2)	0.36	97 (62.6)	64 (58.2)	0.55	99 (65.6)	70 (64.8)	0.99	73 (72.2)	35 (64.8)	0.44	-	-
Used in last 28 days: n, (%)	25 (33.8)	24 (42.9)	0.38	38 (42.7)	27 (45.8)	0.84	41.0%	39 (41.9)	24 (38.7)	0.82	37 (38.1)	25 (39.1)	0.99	27 (26.5)	27 (37.0)	0.19	23 (31.5)	14 (40.0)	0.51	36.0%	0.16
New user since last year: n, (%)	-	-		29 (23.2)	16 (19.0)	0.59	21.5%	19 (21.8)	9 (13.6)	0.28	9 (14.5)	7 (13.2)	0.99	4 (8.2)	5 (12.5)	0.73	5 (16.1)	1 (5.6)	0.39	14.5%	0.037

Table 3:

Results of Time Series Analyses

Factor	Proportion of participants who ever used (%)		Proportion of ever users who used in the last 28 days (%)		Mean attitudes score of participants		Mean intention score of participants1
	Coefficient	p-value	Coefficient	p-value	Coefficient	p-value	Coefficient	p-value
Intercept in WI	33.4	0.007	34.0	<0.001	1.99	<0.01	0.78	0.10
Slope in WI pre-legalization	13.9	0.10	8.3	<0.001	0.42	0.004	0.55	0.34
Difference in intercept of WA vs. WI	3.1	0.53	9.0	<0.001	−0.20	0.014	−0.33	0.48
Difference in pre-legalization slope between WA vs WI	−5.8	0.49	−5.6	<0.001	0.00	0.97	0.05	0.94
Immediate effect in WI at time of legalization	−3.6	0.68	−8.0	<0.001	−0.15	0.08	0.00	0.99
Difference between pre- and post-legalization slopes in WI	−9.5	0.17	−14.6	<0.001	−0.27	0.01	−0.60	0.29
Difference in immediate effect at time of legalization between WA vs. WI	3.5	0.78	−1.4	0.002	0.10	0.25	0.10	0.93
Difference between pre-and post-legalization slopes in WA vs. WI, i.e. difference in differences of the slopes	6.4	0.43	11.5	<0.001	0.02	0.72	0.00	0.99

¹Intention to use marijuana in the next six months; WA, Washington; WI, Wisconsin

Mean attitudes towards marijuana were also similar between the states at all time points (Table 2). For example, in the summer following legalization, the mean attitude score was 2.6 in Washington and 2.7 in Wisconsin (p=0.56). The composite measure for mean attitude across both states was significantly higher after legalization than before, rising from 2.1 pre-legalization to 2.8 post-legalization (p<0.001).

Conversely, the mean score representing intention to use marijuana in the next 6-months was significantly higher in Wisconsin than Washington during the first summer of the study (0.9 versus 0.4, p<0.001, Table 2). However, this intention increased in both states in the following years, such that there was no subsequent difference in scores. Composite measures of the mean intention to use across both states rose from 0.9 pre-legalization to 1.8 post-legalization (p<0.001).

When time series analyses were conducted to compare mean attitude and mean intension to use across the two states, neither measure changed significantly more in Washington after legalization than in Wisconsin (Table 3). However, there were significant differences between the two states in terms of practices among the subset of participants who had ever used marijuana. Among participants who had ever used marijuana, on time series analysis, there was a significant difference between Washington and Wisconsin with regards to the difference in pre- and post-legalization slopes for the 28-day use outcome (Table 3, Figure 2). Among these participants, the proportion that reported using in the last 28 days rose significantly faster in Washington after RML than it did in Wisconsin. This occurred, despite the fact that the proportion of ever users reporting use in the last 28 days was similar in both states at all time points (Table 2).

Figure 2:

Changes over time in use, attitude and intention towards marijuana

DISCUSSION

The conflicting effects of RML on the attitudes, intentions, and behaviors of college students, highlight the complexity of these interactions. While the proportion of participants ever using remained similar across states overtime, the prevalence of 28-day use increased significantly more in Washington after RML than in Wisconsin. Thus, rather than increasing the prevalence of new users, our findings suggest that legalization had the greatest direct effects on current marijuana users, who were surrounded by a climate that was increasingly supportive of its use. The disproportionate increase in monthly use after legalization is consistent with Miller, et al.’s recent Washington study, in which average 30-day use among college students at Washington State University increased by approximately 0.5 additional days, post-RML [7]. Of note, they found no additional increase in average 30-day use after dispensaries opened in 2014.

Given the disparate trends in our study of 28-day use in Washington and Wisconsin during the post-legalization period, it is not surprising that these effects cancel out such that there is no difference in pre- and post-legalization rates when the data from both states were considered together. However, the average rates of 28-day use reported by post-graduation participants in later interview cycles are much higher than expected from previous studies [14–16]. Arria et al.’s longitudinal study started with a cohort of incoming college freshman in 2004, eight years before recreational marijuana became legal anywhere in the United States. Among the six groups of marijuana users that she describes, 61.1% of the sample (71.5% of the weighted sample) was categorized as non-users, corresponding to consistent negligible use over time [16]. In contrast, our study found that 40.0% of participants from Washington and 31.5% from Wisconsin had used in the past 28-days, even at the interview time point one year after college graduation. These rates closer approximate the proportion of participants at the same time point in Arria, et al.’s cohort that report using marijuana in the past year [14].

Similarly, the proportion of participants who report having ever used marijuana was ultimately higher in our study than in Arria et al.’s [14]. Although the proportion of participants who had used prior to college was higher in Arria et al.’s cohort than our study (49.7% vs 34.0%), the rate of ever use was as high in our final cohort year, one year after graduation (67.7%), as it was in the Arria cohort two years after graduation (67.9%; [14]). This may be due in part to expected dwindling rates of new use initiation in the years post-graduation. Yet, while the incidence of marijuana initiation in the Arria cohort slowed after graduation [14], the incidence of marijuana initiation in our study remained nearly as high in the state of Washington immediately after graduation as it was after participants’ sophomore year (12.5% vs 13.6%).

The post-RML increases in scores that we found reflecting average attitude towards marijuana and intention to use in the next six months are also striking. While intention to use is expected to peak during college and decline after graduation, we found a sustained increase in these measures after legalization as participants progressed through college and graduated. During our final interview one year after graduation, attitude and intention to use scores were on par with these participants’ scores during their sophomore year. This phenomenon occurred similarly in Washington and Wisconsin, thus is likely related more to a nationwide change in the discussion and climate for legalization, rather than a direct effect of legalizing possession. It is important to note that these changes occurred despite the fact that for much of the post-legalization period, participants were younger than the legal marijuana age of 21 and that opening of dispensaries did not occur in Washington State until July 2014.

This study was limited by the data collection timeline of the larger longitudinal study. While RML passed in November 2012 and took effect in December 2012, the first recreational marijuana dispensaries were not opened in Washington State until July 8, 2014. Thus, we may have missed changes in the epidemiology of marijuana use that occurred further downstream of our final interviews. In Miller, et al.’s study of the effects of RML at Washington State University, there were no additional changes in 30-day use after the dispensary openings in 2014 [7]. Nevertheless, we sought to mitigate these concerns by adding an extra sixth year of data collection in 2016, onto the initial five-year study.

Potential under-reporting of marijuana use, and thus misrepresentation of actual practices, is another study limitation. Fostering an environment that ensures confidentiality is essential to collecting reliable information, especially in the context of questions regarding illegal activities. We aimed to optimize conditions for honest reporting by including extensive language regarding confidentially to participants at the beginning of every interview cycle.

Finally, as with other analytic methods, small sample size may have affected our ability to detect statistically significant changes in the time-series analysis results. The clinical significance of changes in two of the three non-significant time series results were negligible. Mean attitude towards marijuana increased by only 0.02 points/year and average intention to use in the next 6 months was stable at 0.00 points/year. Thus, even if these measures were inappropriately deemed not statistically significant due to insufficient sample size, the policy implications of these findings are trivial. The effect size of the difference between states regarding pre- and post-legalization slopes of the ever use measure is larger, at 6.4% per year. Although current use is likely more important from a public health standpoint that ever use [3], marijuana initiation is still an important health outcome and additional work should be conducted to further evaluate these effects.

This is the first analysis of its kind to longitudinally evaluate the effects of RML on the attitudes, intentions, and use behaviors of college students. As such, it provides much needed guidance regarding the impact of these policies on young adults. Our findings suggest that legalization likely has the greatest effects on current marijuana users, with lesser implications for one-time, rare, or occasional use.