. Exposure to green space may affect the cardiovascular system by way of mitigating harm (e.g., air and noise pollution and heat), restoring mental capacities (e.g., attention restoration or stress recovery), or building capacities (e.g., physical activity, social connectedness) [3
]. The majority of between- and within-subject design studies measured cardiovascular health outcomes. Gidlow et al. [58
], Song et al. [70
], Song et al. [69
], and Song et al. [71
] conducted experimental studies using a within-subjects study design in which all subjects were exposed to both green space and control environments. For all studies, participants were asked to first walk for a set amount of time (between 15 and 25 min) in an urban green space, and then that same amount of time in an urban built space. All of these studies except Gidlow et al. [58
] measured heart rate (HR), and all four studies measured heart rate variability (HRV).
. Experimental studies using a between-subjects design divided participants in to groups and exposed each group to a different environmental exposure [43
]. Brown et al. [57
] and Grazuleviciene et al. [76
] measured physiological responses to a long-term repeated exposure both using random group assignment (Brown et al. [57
]: two 20-min walks per week for 8 weeks; Grazuleviciene et al. [76
]: 30 min of walking for 7 days), while Hartig et al. [43
] exposed participants to 40 min of walking or sitting. Control environments were built urban areas, indoor seating [43
], or residential area with unmitigated vacant lots [50
]. Hartig et al. [43
] and Grazuleviciene et al. [76
] took stationary measurement of HR while participants were seated or resting, before and after exposure. South et al. [50
] is the only study to conduct ambulatory measurements of participants’ HR before and after a place-based intervention was conducted (vacant-lot greening). Brown et al. [57
] took pre- and post-intervention measurements of BP, HR, HRV as well as autonomic function, risk of cardiovascular disease (CVD), and aerobic fitness.
. Most experimental studies measuring HR response found lower HR in urban green space compared to control environments. Song et al. [71
], Song et al. [70
], and Song et al. [69
] found lower HR when subjects walked through an urban park compared to a built urban setting. However, these studies had small sample sizes, and order of study site visit was not randomly assigned. South et al. [50
], though also based on a small sample and not controlling for individual characteristics that might influence stress response, found a significant decrease in HR for participants that walked by cleaned-and-greened vacant lots. Grazuleviciene et al. [76
], based on a small sample, found that the group that walked in an urban park had greater reductions in DBP and HR, and increases in HR recovery when comparing pre- and post-measurements. Hartig et al. [43
] found no statistical difference in BP or HR. Brown et al. [57
] found no significant differences in any cardiovascular outcomes.
Measurement of, and findings on HRV have been mixed. Four studies found that HRV was significantly lower in green environments compared to in control environments using pre-post comparisons [69
]. Significant findings of HF component were in a positive direction [69
], while none that measured LF component found significant (positive) differences. Out of the four studies that measured LF/HF component, three studies found no difference [69
], and one found significant negative difference [69
]. Brown et al. [57
] and Gidlow et al. [58
] found no statistical difference in any HRV indicators.
While studies in general found a negative association between urban nature exposure and HR, these studies were predominantly based on small sample size and did not control for confounding factors. Few consistent measures of HRV have been used, in a small number of studies, and findings are mixed. These findings are bolstered by an increasing number of longitudinal cohort and quasi-experimental studies.
Cardiovascular health outcomes were measured in two longitudinal cohort studies. Paquet et al. [74
] found no association, while controlling for individual-level demographic factors, between residential green space (using an aggregated construct) and incidence of hypertension or dyslipidaemia among a cohort of adults living in two metropolitan regions of Adelaide, South Australia. Tamosiunas et al. [78
] measured the association between green space exposure (distance from residence to nearest park, and self-reported park use) and markers of CVD among a cohort of residents (ages 45–72) of Kaunas City, Lithuania. This study was able to control for both individual-level demographic and health behaviors (smoking and physical activity). They found an increased risk of fatal and non-fatal CVD for participants (especially men) that lived further from green spaces; and increased risk of non-fatal CVD among non-park users that lived further from green spaces.
Two quasi-experimental studies have examined the association between urban nature-based interventions and change in cardiovascular outcomes. Branas et al. [13
] conducted a quasi-experimental study of changes in neighborhood crimes and health outcomes (including rates of hypertension and high cholesterol) that occurred near clean-and-green interventions to vacant lots relative to near untreated vacant lots. They found some significant decreases in high cholesterol across the city near treated lots. In a study of changes in incidence of high blood pressure and high cholesterol near green stormwater infrastructure sites compared to near control sites, Kondo et al. [36
] found no changes.
3.6.4. Mental Health
. Aspects of brain function, including cognitive development and attention restoration, may be pathways by which environments affect mental health. Six studies tested the association between urban nature exposure and measures of cognitive function. There was a general finding among five studies in this category [17
] that exposure to urban nature compared to urban built environments improved multiple measures of cognitive function or development, including attention or attentional capacity and working memory.
Five studies used an experimental approach to measure changes in attention in response to a walk in urban nature versus a walk in a built urban setting [37
]. Bratman et al. [37
] found negative associations between rumination and subgenual prefrontal cortex activation and exposure environment (90-min walk in urban nature preserve versus built urban walk) and by time (pre- post-comparisons). Participants were urban residents. In a between-subjects study of the association between nature exposure, mood, and attentional capacity [47
], participants that walked in urban nature preserve had improved attention and ability to reflect on a life problem, relative to those who walked in an urban built area.
Gidlow et al. [58
] asked participants to take 30-min walks in natural (green), natural with water (blue), and residential control (urban) environments, and took measurements of psychological and physiological stress at baseline, at the end of the walk and 30-min after the walk. They found improved attention and restoration only in the green and blue environments.
Other studies measured nature exposure via residential location, finding positive effects in general. Dadvand et al. [17
] and Kuo [46
] measured association between green space characteristics at participants’ residence and attention. Kuo [46
] found less attention fatigue among public housing residents assigned to live in housing blocks without nearby nature compared to among those assigned to live in blocks with nearby nature. Dadvand et al. [17
] measured association between green space exposure and aspects of cognitive development including working memory and inattentiveness based on repeated measurements over a 1-year period among a cohort of children (ages 7–10) in Barcelona. They found that adding traffic-related air pollution concentrations to statistical models helped explain 20–65% of the estimated positive association between green space (using a composite index) and attention. This is the only study to consider effect modification of air pollution exposure. Hartig et al. [43
] found no difference in attention between nature exposure and control groups.
In general, experimental study design has been used to assess the relationship between cognitive function, primarily attention, and green space exposure. While sample sizes have been small and non-random, and studies are subject to biases associated with within- and between-subject designs, a positive association has been found with nature exposure.
. Six studies examined the association between self-reported measures of general health and green space exposure. These studies used standardized instruments such as the General Health Questionnaire [55
], or the General Health Perception Scale [51
]. The General Health Questionnaire was used to measure either “general health” [55
] or “mental health” [53
]. Half of these studies found positive association between green space exposure and generalized measures of health, while the other half found no association.
Astell-Burt et al. [55
] used general health questionnaire to indicate mental health in a British Household Panel Survey with nine annual waves. While adjusting for a wide range of individual-level demographics and health behaviors, they found that association between availability of green space and mental health increased in significance and magnitude for both men and women (depending on level of green space) as they aged. Alcock et al. [53
] also used data from the British Household Panel Survey between 1991 and 2008, of participants that moved, introducing a quasi-experimental research design. For each participant Alcock et al. [53
] analyzed five annual waves; two years prior and three years after relocation. No control participants were included. Controlling for individual and community-level demographic factors, they found that mental health was significantly improved throughout the post-relocation period among participants that relocated to greener areas.
Brown et al. [57
] used a questionnaire to assess participants’ general, mental and physical health. Self-reported mental health improved for the nature walk group compared to the control group that took two walks in a built urban setting. Droomers et al. [61
] found no difference in self-reported general health when comparing pre-post measurements among residents of neighborhoods that received green space improvements and residents of neighborhoods that did not receive green space improvements. However, the greening improvement was not standard across intervention neighborhoods, and assessment of improvements was assessed via interviews, which could explain a lack of difference in exposures in intervention and control neighborhoods. Tamosiunas et al. [78
] and Wolfe et al. [51
] also found no association.
Mood and Emotion
. Eighteen studies have investigated whether nature exposure relates to mood, using various measures (see Table 2
). Most of these studies have found evidence of improved mood or emotion after nature exposure, compared to after exposure to a built urban environment.
While many of these studies were motivated by the question of whether nature exposure could be used as an effective psychological intervention on clinically-relevant mental health disorders such as depression and anxiety, only one study tested change in mood after therapeutic interventions with a clinical population specifically. Barton et al. [56
] recruited persons with a health precondition (mental illness). They found that both self-esteem and mood were significantly improved in the green exercise group (compared to control groups), with a dose-response relationship of time.
Most other studies measured change in mood after exposures obtained during walks of varying length and frequency, in both urban nature and control environments. Mood improved in nature exposure groups compared to in groups that walked in built urban environments [47
], walked in indoor settings [64
], relaxed in an indoor setting [43
], or did anagram puzzles [67
]. Brooks et al. [64
] added the dimension of season to their walking study, finding no effects. Aspinall et al. [54
] was the only study to use mobile electroencephalography (EEG) to monitor emotional experience during experimental exposure process. The study found that participants had more positive emotions and less negative emotions when navigating through urban green spaces compared to built urban spaces.
Instead of an outdoor walk, van den Berg and Custers [62
] subjected participants to a stress task and then exposed them to either 30 min of outdoor gardening or 30 min of indoor reading. Although based on a small, non-random sample, they found that positive mood was improved in the outdoor gardening group only.
Semenza et al. [49
] conducted a pre-post study of greening interventions in three neighborhoods. They collected pre- and post- survey measurements of mental health, sense of community and social capital from residents living two blocks from the interventions. Although the study had no control group, they found improvements in all three measures.
Few studies have tested the relationship between mental health and quality of green space exposure. Tyrväinen et al. [79
] examined the psychological (restoration and mood) effects of visits to urban environments (urban park, urban woodland or built urban). Restoration and mood improved in both nature settings, but restoration was more improved in the urban woodland. Carrus et al. [80
] found an association between biodiversity of green space and well-being, mediated by length of park visit and perceived restorativeness.
Three studies found no difference in association between nature- and control-group exposures and mental health. Butryn and Furst [38
] measured mood and feeling states before and after participants, female distance runners, went on a 4-mile run on a natural and a built urban route. In general, they found that though participants preferred the urban park run, mood and feeling states improved post-run in both settings. Hull and Michael [44
] asked participants to spend leisure time in an urban park and then in their home, and found improved mood after leisure time in both locations. Grazuleviciene et al. [76
] found that negative affect did not change for the nature walk group.
Compared to studies of cognitive function outcomes, a wider variety of study designs have been used to assess the relationship between urban green space exposure and mood. Although mood has been measured using an inconsistent set of scales and concepts, in general a positive association has been found.
. Six studies examined the association between green space exposure and depression or depressive symptoms and produced mixed results. Gubbels et al. [20
] and Semenza et al. [49
] were both greening intervention studies, and both used the Center for Epidemiologic Studies Depression Scale (CES-D). Gubbels et al. [20
] conducted a two-wave longitudinal study of changes in walking and cycling frequency associated with neighborhood green space interventions and perceived greenery. This is one of few studies to focus on “deprived” neighborhoods. They found that perceptions of greenery improvements were associated with a decrease in depressive symptoms among adults. Semenza et al. [49
], though under-powered, found no difference in depression scale between intervention and control sites.
The remaining studies tested mental health response to short-term nature exposure (compared to built urban or indoor exposure) through nature walk or run. Butryn and Furst [38
], Song et al. [70
], and Song et al. [69
] used the Profile of Mood States (POMS) questionnaire, and found a negative association between urban green space exposure and depression. While Brooks et al. [64
] used the short-form version of the Depression and Anxiety Stress Scales (DASS-21) and found significant time and season effects.
. Stress response to urban nature exposure has been measured using psychological and physiological measures. Three studies measured the association between urban green space exposure and psychological stress. All these studies indicate a positive association. Using a within-subjects study design, Beil and Hanes [36
] found significant improvements in post-exposure measurements of self-reported stress for participants (especially women) exposed to natural urban settings compared to built settings. Branas et al. [13
] also found improvements in self-reported stress among residents living near cleaned-and-greened vacant lots compared to residents living near control untreated vacant lots. Kondo et al. [36
] found no difference in self-reported stress among residents living near green stormwater infrastructure projects compared to residents living near wait-list control sites.
There is mixed evidence of an association between urban nature exposure and physiological stress response. Cortisol concentration, measured either by saliva or blood, is a common marker of physiological stress, and was used in five experimental studies [36
]. It should be noted that none of the studies measured diurnal cortisol concentration. In general, cortisol saliva samples were taken before and after outdoor exposures, though some studies also collected samples during the exposure [62
]. Most of these studies found no significant difference between pre- and post- exposure measurements of cortisol [36
]. Tyrväinen et al. [79
] is one study that examined both the physiological (cortisol concentration) and psychological (restoration and mood) effects of visits to urban environments (urban park, urban woodland or built urban). Cortisol concentration decreased in all three settings. One study [62
] found that cortisol concentrations were improved with urban green space exposure (in the outdoor gardening intervention group and not in the indoor reading control group).
. Only one study examined the association between nature exposure and social, emotional and behavioral problems. Richardson et al. [33
] conducted a longitudinal cohort study of children between 2005 and 2010. Participants were approximately 1 year old at recruitment. Using the Strength and Difficulties Questionnaire, and controlling for many individual, family and neighborhood factors, the study found that lack of private access to gardens was associated with hyperactivity-, peer-, conduct-problems and total difficulties, while increasing exposure to green space (measured as percent green space and parks within ward of residence) was associated with improved social outcomes.
Urban green space exposure could impact weight status or other measurements of the metabolic system via cardiovascular, mental health and physical activity pathways. Seven studies measured body-mass index (BMI) [22
] or other measurements of weight status, including abdominal obesity [74
], in association with urban green space exposure.
Only two of these studies found a negative relationship between BMI and urban nature exposure. Bell et al. [24
] conducted a longitudinal cohort study of children ages 3–16 years recruited from a primary care clinic of a health network in Indianapolis, IN. Two BMI measurements were taken at least two years apart and compared with the amount of residential green space (NDVI within 1 km of residence). While controlling for residential density in addition to demographic factors, they found that higher residential green space was associated with lower BMI (z-scores) in the second measurement. Wolch et al. [22
] examined the association between proximity to parks with childhood obesity using data from eight annual survey waves from a longitudinal cohort study of 3173 children in California. While controlling for multiple potential confounding factors, BMI growth at age 18 was inversely associated with park access (park acres within 500 m of residence), more so for boys than for girls.
The remaining five studies found no association between BMI and green space exposure. Brown et al. [57
] found no difference between short-term (8-week) green space exposure and BMI and predicted aerobic fitness. The remaining studies used a longitudinal cohort design. Gose et al. [19
] examined the association between neighborhood social and environmental factors and BMI using a 4-year longitudinal cohort study of children in Kiel, Germany. They found no difference between long-term green space exposure (distance to nearest park/green space within 800 m of residence) and BMI. Michael et al. [80
] found no relationship between neighborhood environment (proximity of park or green space) and BMI among older women over the 18-year study period.
Paquet et al. [74
] found no association between availability/access to public open space (including greenness) and abdominal obesity. However, they found that larger public open space (not necessarily vegetated/green space) was associated with lower risk of developing prediabetes/diabetes. Tamosiunas et al. [78
] found that distance from residence to park space was not associated with BMI, however they found that self-reported park users, compared to non-park users were less likely to be obese (BMI greater than or equal to 30).
The majority of evidence suggests no association between BMI and urban green space exposure. However, none of the studies reviewed explored physical activity, cardiovascular disease or mental health, which are expected to be pathways of association, as effect modifiers.
Studies of the relationship between green space exposure and mortality pose strong evidence of consistent negative association. Six longitudinal studies have been conducted with strictly urban cohorts on the association between mortality and green space exposure [51
]. Although each of these studies was conducted with sub-populations and therefore results should not be generalized, all found that increased green space exposure was associated with reduced odds of mortality. Takano et al.’s [72
] foundational study, a prospective cohort of elderly Tokyo residents, found that having green space and gardens nearby one’s residence increased survival odds. Wilker et al. [51
] conducted a longitudinal cohort study of adults admitted for acute ischemic stroke between 1999–2008. Using death records collected in 2012, they found a lower hazard ratio for stroke mortality among patients with higher green space exposure (NDVI at residence).
Two of these longitudinal studies controlled for ambient air pollution concentrations. Villeneuve et al. [68
] conducted a cohort study of adults living in 10 urban areas of Ontario, Canada between 1982–86. Among participants, higher green space exposure (NDVI within 500 m of residence), while controlling for ambient air pollution, was associated with reduced non-accidental mortality, with strongest association for respiratory disease mortality. However, this study did not control for smoking behavior. Crouse et al. [65
] conducted a large cohort study of mortality among non-immigrant Canadians residing in 30 cities between 2001 and 2011. Using annual measurements of residential green space, they found a protective association with non-accidental, cardiovascular (plus diabetes), cardiovascular, ischemic heart disease, cerebrovascular, and respiratory mortality. Sex, age, income, educational attainment and marriage status modified the estimates.
While not strictly an urban sample, James et al. [45
] used mortality data from a cohort of nurses in 11 states of the USA, 84% of whom were living in urban areas. They found that living within the highest quintile of residential greenness reduced risk of overall mortality by 12%. They found reduced risk specifically for cancer, respiratory and kidney disease mortality, and no association with other causes including coronary heart disease. Stratifying by urban versus non-urban residence did not change estimates, and physical activity, air pollution, and diagnoses of depression modified the estimates.
In a single case-crossover study, Gronlund et al. [41
] examined how individual and neighborhood characteristics (including percent green space) modified the association between extreme heat (EH) events and cardiovascular mortality using death records from eight cities in Michigan between 1990 and 2007. The odds of cardiovascular mortality were higher among individuals living in zip codes with low amounts of green space.
A growing number of longitudinal cohort studies provide evidence of a significant negative association between green space exposure and mortality. However, while there was more than one study examining (and finding negative association for) all-cause (non-accidental), cardiovascular and respiratory mortality, there was only one study examining other mortality causes, including stroke, cancer, cerebrovascular disease, and kidney disease. It should be noted that most of these studies were conducted with sub-populations and did not control for other environmental exposures (e.g., air pollution) that might affect mortality risk.
3.6.7. Physical Activity
Whether or not someone engages in physical activity may be influenced not only by individual characteristics, but also by the accessibility, features, condition, and actual and perceived safety of their surrounding physical environment [91
]. Increased exposure to urban green space could improve health by increasing opportunities and actual physical activity levels. A variety of methods have been used to test the association between green space exposure and physical activity, including momentary/time-activity studies, experimental design (within- or between-subjects), longitudinal studies, and place-based interventions.
Physical activity is most often measured using accelerometer as time spent in moderate-to-vigorous physical activity (MVPA). Twelve studies examined the association between green space exposure and physical activity, measured as MVPA. Nine of these studies focused specifically on youth or adolescents [21
]. All tracked location of MVPA using a combination of wearable GPS and accelerometer, except de Vries et al. [26
] that used activity diaries. Characteristics of momentary locations were typically classified using land use or landcover categories.
In general, these studies found that amount of time spent in MVPA versus not was higher when surrounded by green space (of differing types). Half of these 12 studies calculated likelihood of MVPA using means comparison, and therefore did not control for confounding factors. Coombes et al. [25
] found that the mean number of minutes of MVPA was higher in green space locations than in buildings and paved areas [25
]. Jones et al. [21
] found that among urban participants, MVPA was more common in gardens and streets than in other environments. Moore et al. [31
] found some MVPA in green space, but less than in other domains (school, home, street). Lachowycz et al. [29
] found that up to 30% of MVPA among 10–11-year-olds was done in parks. Klinker et al. [27
] and Klinker et al. [28
] found no significant difference in MVPA for participants when spending time in green space.
The remaining six of these studies used statistical models to calculate odds of MVPA while controlling for individual and community factors. Nearly all these studies found that time spent in urban green space was positively associated with MVPA. Odds of MVPA were higher for study participants when spending time in parks than in other land uses [32
]. de Vries et al. [26
] found that adjusted odds of MVPA were higher with higher quantity of residential green space (using subjective scoring). Almanza et al. [23
] found that MPVA was nearly five times greater among children that spent more than 20 min of time in green space, than among children with no green space exposure.
Finally, Zenk et al. [52
] found no association between presence of park land use and MVPA. It should be noted that none of these time-activity monitoring studies, except Zenk et al. [52
], coded GPS data by activity spaces or transportation modes, which could limit causal inference [92
]. Likewise, instead of MVPA, Grazuleviciene et al. [77
] compared exercise duration between participants that walked in an urban park and participants that walked in a built urban environment, and found no difference.
Longitudinal studies have found positive association between green space exposure and self-report measures of physical activity. Beenackers et al. [73
] found that after residential relocation, participants’ self-reported cycling behavior increased, moderated by self-reported access to parks. Sugiyama et al. [75
] conducted two surveys of physical activity four years apart of a cohort in Adelaide, Australia. They found that walking maintenance was associated with green space exposure (total and largest area within 1.6 km of residential neighborhood). Tamosiunas et al. [78
] found that inactive participants were more likely to be non-park users and to have less exposure to green space.
Five studies evaluated physical activity in response to a greening intervention, and there is mixed evidence of impact. Branas et al. [13
] found a positive association between exercise levels (increase) in residents living near vacant-lot cleaning-and-greening interventions. Thompson et al. [60
] included a small sample, however they found an increase in exercise frequency among surveyed residents of a greening intervention neighborhood compared to control area. In addition, Gubbels et al. [20
] conducted a two-wave longitudinal study of changes in walking and cycling frequency associated with neighborhood green space interventions and perceived greenery. This was one of few studies to focus on “deprived” neighborhoods. They found that residents of greening intervention neighborhoods had a smaller decrease in cycling among adolescents compared to residents of control neighborhoods.
The remaining studies found no impact. Using five years of cross-sectional surveys Droomers et al. [61
] found no change in physical activity among residents of intervention neighborhoods compared to among residents of control neighborhoods. Although conducted with a small sample, Gustat et al. [42
] found no difference in rate of physical activity (including MVPA) among residents of a single greening intervention neighborhood and two control neighborhoods.
A wide variety of methods have been used to test the association between green space exposure and physical activity. While in general, experimental (within- or between-subjects) and longitudinal studies have found a positive association between urban green space exposure and physical activity, findings from place-based intervention (observational) studies have found no association.
Studies that have examined the association between urban green space and violence were mostly quasi-experimental, with analysis of change in violence after a greening intervention at both intervention and control locations. Violence-related outcomes included crime, feelings of safety and aggressive behaviors. Three studies that measured changes in crime found significant decreases after place-based green space interventions occurred [13
]. In a study of changes in crime outcomes near 4,436 cleaned-and-greened intervention vacant lots compared to near 13,308 control lots, Branas et al. [13
] found reduced gun violence, though no difference in nuisance or drug-alcohol crimes, near greened vacant land compared near blighted vacant land. Kondo et al. [14
] examined the association between changes in crime around 254 intervention cleaned-and-greened (some by community members and some by city-contractors) vacant lots in Youngstown, Ohio compared to around 959 control lots. The study found a reduction in all crimes around intervention lots, specifically in property crimes around contractor-greened lots and in violent crimes around community reuse lots. In addition, Kondo et al. [36
]’s quasi-experimental study of the association between green stormwater infrastructure installation and crime did not find any significant effects on violent crimes but did find reduced narcotics possession arrests around greened sites compared to control locations.
A single study used the removal of trees as a natural experiment of the relationship between green space and crime. Kondo et al. [16
] used quasi-experimental design to examine the change in crime in Cincinnati after the emerald ash borer decimated the ash tree population and found an association between removals and Census-block levels of both violent and property crimes. In addition, there has been one experimental study, a small-sample randomized controlled trial of violence outcomes associated with a vacant lot greening intervention [40
]. After the intervention, residents living near intervention lots felt safer compared to residents living near lots that were left vacant. All these studies featured greening interventions (and controls) occurring primarily in low-income neighborhoods.
Using both a case-control and case-crossover technique, Kondo et al. [30
] examined the presence versus absence of tree cover for 1-min activity pathpoints on the day of assault, and its influence in risk of assault. They found that being under tree cover reduced odds of gun assault using case-control and case-crossover methods, especially in low-income neighborhoods.
In addition, Younan et al. [35
] examined the association between violent behavior (aggression) and urban green space exposure (average NDVI surrounding residence) using a longitudinal cohort study with four waves. They found that increased exposure to green space was associated with reduced aggressive behaviors.
Seven studies have examined the association between urban green space and violence outcomes, and most have been quasi-experimental studies comparing pre- and post-intervention levels of crime or violence between treatment and control groups. Although nearly all are lacking true randomization, all, but one, found significant negative relationships.