“Happiness is the meaning and the purpose of life, the whole aim, and the end of human existence.” The ancient Greek philosopher Aristotle said these words more than 2000 years ago, and they still ring true today. The 1776 U.S. Declaration of Independence states that all men have a right to “the pursuit of happiness,” where the notion of happiness is equated to the attainment of a worthy life ^[1][4].

Moreover, the concept of happiness is gaining increasing popularity within and across cultures ^[2][3][4][5,6,7], so much so that in recent years, there has been a shift in measuring economic production to measuring happiness as an indicator of social development and individual welfare across nations ^[5][8].

Since the advent of the field of positive psychology in the late 1990s, scientific investigations have uncovered happiness as an essential psychological ingredient for optimal human functioning that makes life worth living ^[6][9]. Happiness is conceptualized as an appraisal of life ^[5][8], a state of mind ^[3][6], a psychological state ^[2][5], and a positive health indicator ^[5][8], and is synonymous with subjective well-being ^[4][6][7,9]. All in all, happiness has been defined in various ways.

Happiness as overall satisfaction with life: Happiness has been conceptualized as an evaluation of life ^[5][8], as overall satisfaction with everyday life ^[7][10], and as the overall quality of one’s life ^[8][11].

Happiness resulting from positive experiences and positive outcomes: Traditionally, happiness has been defined by the experience of more frequent positive affective states than negative ones ^[9][12]. Happiness is more than just a personally important goal or a set of pleasant mood states ^[10][13], and is related to, precedes, and causes a variety of favorable life outcomes ^[11][14]. Furthermore, across studies, happiness has been defined as a positive subjective experience ^[12][15].

Happiness as a psychological state of mind, and well-being: Studies have also defined happiness as a psychological state ^[2][5], a state of mind ^[3][6], a “state of being” ^[13][16], a positive attitude toward life ^[14][17], a healthy mental status, emotional balance, hope for the future ^[14][17], and subjective well-being (SWB) ^[4][7], which is the psychological state of well-being, joy, and contentment ^[11][15][14,18]. Happiness as an emotional state is linked to one’s physiological reactions to life events ^[16][19] based on the Hedonic Adaptation Theory of Brickman and Campbell ^[17][20] and the Set-Point Theory ^[18][21].

Happiness as a health indicator: In a study, 785 participants were asked to list associations that came to mind on hearing the word happiness ^[19][22]. The participants associated happiness mostly with health and relationships.

Happiness as a transient state: Happiness is defined as a transient mood state of enthusiasm and joy, and it reflects the person’s effect on one’s current state ^[12][15].

Despite the myriad conceptualizations of happiness, there are several questions that remain unanswered. The research questions that guided this inquiry were: Is happiness a temporary state of mind or emotion? Is happiness something we are born with, attain with time, or both? Or Is happiness a period of long-term life satisfaction and general well-being that we all aspire to have in our lives?

2. Health and Happiness

2.1. Mental Health and Happiness

A total of nineteen studies examined the effects of mental health on happiness. The age range of the participants across these studies was between 10 and 99 years, and the participants were from China, Europe, Germany, India, Iran, Korea, New Zealand, Romania, Spain, Thailand, Turkey, the UK, and the USA. These studies used both female and male participants where a majority were male (55% female, 45% male). Thirteen studies showed an increase in happiness caused by mental health determinants such as positive general mental health, mindfulness, decreased posttraumatic stress symptoms, creativity, and self-affirmation. Six studies showed a decrease in happiness caused by the determinants of mental health such as adverse mental health outcomes, depression, poor perception of health, lifetime trauma, addiction, and heavy use of screen-based media. These determinants were seen to have a negative relationship with happiness across Asian, American, Black, Hispanic, Native American, Mixed, and White cultures and ethnic groups. Several studies investigated mental health treatment and its relationship with happiness. A group of researchers showed that meditation (seven-day intensive Vipassana retreat) enhanced happiness ^[20][34]. Another research study found that a greater number of sessions per client and decreased post-traumatic stress symptoms were associated with greater counselor happiness ^[21][35]. Mindfulness, grit, and coping competence were found to positively predict happiness ^{[6][22][23][24]}[9,36,37,38]. Hope and mindfulness were found to share a positive relationship with happiness, and the recognition of new possibilities and personal strength predicted happiness ^[25][26][39,40]. Another group of researchers showed that engaging in spontaneous self-affirmation was related to greater happiness and that self-esteem is an antecedent of happiness ^[27][28][41,42]. Several studies examined negative determinants of happiness. A research study found that a poorer perception of mental health was associated with less happiness ^[29][43]. Another study showed daily subtle negative experiences were related to adverse mental health outcomes, such as depression, suicidality, and decreased happiness ^[30][44]. A study that examined alexithymia, depression, anxiety, stress, and the relationship of fatigue with happiness, found that decreased posttraumatic stress symptoms were associated with greater counselor happiness ^[31][45]. Higher rates of current depression were associated with higher levels of happiness seeking, and greater distress (behavioral health) was associated with lower global happiness ^[32][46]. Research showed depression was significantly and negatively associated with pleasure ^[33][47], which in turn is associated with happiness. Research revealed an association between creativity and depression and happiness ratings ^[34][48]. Other studies examined traumatic life events and happiness. The relationship between lifetime trauma and happiness found that bereavement of a child was associated with lower levels of happiness ^[35][36][49,50]. A negative relationship between stressful life events and happiness was found among humbler people ^[37][51]. Studies that examined the association between addiction and happiness found that heavy screen-based media use was associated with less happiness ^[38][52], and higher addiction led to lower levels of happiness ^[39][53]. A study found internet addiction significantly related to subjective unhappiness ^[15][18].

2.2. Emotional Health and Happiness

A total of nine studies examined the effects of emotional health on happiness. The age range of the participants across these studies was between 9 and 64 years, and the participants were from Asia, Africa, Australia, Canada, China, Europe, India, the Middle East, the UK, and the USA. These studies also used both female and male participants (50% female, 50% male). All nine studies showed an increase in happiness across countries caused by myriad emotional health determinants that included psychological well-being, Big Five personality traits, humor, gratitude, efficacy, caring climate, and positive emotions. These determinants were seen to have a positive association with happiness. A study found that psychopathic personality traits such as fearless dominance positively correlated with higher durable happiness and negatively correlated with fluctuating happiness ^[40][54]. Fluctuating happiness was described as a sudden increase in happiness, followed by a sudden decrease ^[40][54]. Big Five personality traits of extraversion, agreeableness, neuroticism, conscientiousness, and openness to experiences were found to be associated with subjective happiness ^[41][33]. Holistic wellness and resilience were found to be determinants of happiness ^[41][42][33,55]. Similarly, psychological well-being was found to have a significant positive association with subjective happiness ^[43][56]. Research showed a positive association between positive emotions and greater happiness ^[44][57]. Positivity predicted positive emotions with greater happiness ^[11][14]. Efficacy and a caring climate were positively associated with happiness (emotional health) ^[45][58]. Adaptive humor styles (affiliative humor and self-enhancing humor) significantly predicted subjective happiness, whereas maladaptive humor styles (aggressive humor and self-defeating humor) did not strongly predict subjective happiness ^[46][59]. Gratitude practice was found to bolster happiness ^[46][59]. Various studies investigated psychological determinants of subjective happiness. Three positive psychology determinants that included gratitude visits, three good things in life, and using signature strengths in a new way, were found to increase happiness ^[47][28]. Positive psychological intervention improved happiness of patients undergoing in vitro fertilization as a treatment to become pregnant ^[48][60].

2.3. Physical Health and Happiness

A total of 28 studies examined the effects of physical health on happiness. The age range of participants across these studies was between 5 and 100 years, and the participants were from 44 countries including Africa, Canada, 15 European countries, the Far East, France, Germany, Georgia, Greece, India, Iran, Italy, Netherlands, Nicaragua, Palestine, Poland, South America, Taiwan, the UK, and the USA. These studies also used most male participants (45% female, 55% male). A total of 16 studies showed an increase in happiness caused by various physical health determinants that included regular physical activity, general physical health, the health of parents, a healthy diet, health insurance, cochlear implantation surgery, and home dialysis, and nine studies showed a decrease in happiness caused by various determinants of physical health such as poor health, disability, handicap, abuse, advancing age, disfigurement, transition to adulthood, older transgender youth, perceived illness, and health problems. A study by a group of researchers showed that general health is associated with general happiness ^[49][61]. Similarly, health was found to positively associate with happiness ^[19][22]. Physical activity was associated with higher levels of happiness ^[50][62] and increasing the volume of physical activity was found to be associated with higher levels of happiness ^[51][63]. Another study found that individuals who are more physically active are happier ^[52][64]. A study also found that regular physical activity was associated with greater happiness ^[53][65]. Some studies examined severe disability and illness with happiness. A group of researchers found increased levels of perceived illness to be significantly associated with decreased happiness ^[54][66]. More health problems and greater perceived seriousness of the health problems/effects were found to be associated with less happiness ^[55][67]. Disability was found to be associated with moderate to large drops in happiness over time ^[56][68]. Positive meta-stereotype (positive image) and better perceived general health were associated with higher overall happiness, whereas feelings of loneliness and disability/handicap were associated with lower overall happiness ^[57][69]. Suffering from a severe disability was associated with less happiness, and higher BMI was associated with steeper declines in happiness ^[58][70]. Conversely, greater happiness was also found among handicapped youth vs. control handicapped youth ^[13][16]. People with disfigurements were subjectively judged as being less happy ^[59][71]. Several studies investigated medical health policies and the perceptions of health and their relation to happiness. A study showed that national health insurance significantly increased happiness ^[60][72]. Cochlear implantation surgery was found to increase happiness in mothers of children with hearing loss ^[61][73]. Higher levels of happiness were found among home dialysis patients ^[54][66]. Another study reported greater happiness post-renal transplant ^[62][74]. Use of a microswitch-based program for Rett syndrome (promotes locomotion fluency) was found to increase happiness ^[63][75]. Some studies established relationships between age and happiness in general and based on early trauma and stressful events experienced throughout life. Some studies found no significant difference in the happiness levels between children, adolescents, and adults ^[7][64][10,76]. Another study showed individual happiness determined by age and found a U-shaped relationship between age and happiness ^[65][77]. Studies also found a trend in the trajectory of happiness from early adulthood to midlife ^[5][66][8,78]; they showed that older adults who experienced traumatic events during childhood vs. after the transition to adulthood exhibited lower subjective happiness; as age increased, happiness levels decreased. Transition to adulthood exhibited lower subjective happiness and happiness showed a downward trend in the older age groups ^[67][68][79,80]. Research showed a negative association between a past-negative time perspective and happiness with aging ^[69][81]. A study found that older transgender youth experienced lower happiness than younger patients ^[70][82]. Being younger, widowed, or separated from a spouse and experiencing high levels of stress had significant direct effects on diminishing happiness with low levels of health satisfaction ^[71][83]. Another study showed that eudaimonia and hedonic happiness remained relatively stable across the lifespan only in the most affluent nations ^[72][84]. This showed the role of determinants in the relationship between age and happiness.

3. Hope and Happiness

Hope was an emergent happiness theme. A total of 23 studies examined the hope-based determinants of happiness, classified into the categories of purpose and goal achievement, personal growth, and economic growth, based on the patterns that emerged across the studies. Within these categories, goal achievement, task performance, a greater set of goals to pursue, the enjoyment of and success at work, life satisfaction, and positive thinking about the future had a positive association with happiness; socioeconomic status, economic scarcity, and unemployment had a positive association with happiness.

3.1. Goal Achievement and Happiness

A total of 10 studies identified a range of work- and study-related determinants that influenced happiness through hope for goal achievement. The age range of participants was between 15 and 94 years, and the participants were from 32 countries including Asia, China, Europe, Germany, Iran, and the USA. Both female and male participants were present in these studies, with a majority being female (55% female, 45% male). All 10 studies showed an increase in happiness caused by the determinants of goal and purpose that included occupation, task performance, goal focus, a greater set of goals to pursue, education, the enjoyment of and success at work, occupational control, compensation, scholastic achievement, self-employment, job training, and need-supplies across different communities. Mastery-approach goals were found to facilitate higher levels of happiness with task performance than performance-approach goals in conditions of unfavorable social comparisons ^[73][85]. Greater elective selection (choosing a particular goal or set of goals to pursue), loss-based selection (selecting goals in the face of resource loss), optimization (enhancing or acquiring resources to achieve a goal), and compensation (reallocating resources towards another goal to maintain functioning at a specific level) were found related to greater happiness ^[74][86]. A person’s valence success at a task predicted greater happiness when they succeeded, but greater unhappiness when they failed ^[75][87]. High core self-evaluation and needs-supplies fit (congruence between employees’ needs and the rewards received for work) significantly predicted greater happiness ^[76][88]. Enjoyment of and success in work and serious hard-working living were determinants contributing to happiness ^[77][29]. Job training, cognition, health, social network, and extraversion explained a substantial proportion of variance in happiness ^[78][89]. Higher occupational status corresponded to greater happiness ^[79][90]. Similarly, more education, higher personal income, and greater occupational control were related to increased happiness (in men) ^[80][91]. Job satisfaction in self-employed workers vs. organizational workers is related to greater happiness ^[81][92]. Nations with better scholastic achievement performances (mathematics, reading, and scientific literacy) displayed higher happiness scores ^[82][93].

3.2. Personal Growth and Happiness

A total of eight studies examined the effects of personal growth on happiness. The age range of the participants across these studies was between 18 and 91 years, and the participants were from Ghana, Slovenia, Switzerland, and the USA. Both female and male participants were present in these studies, with the majority being female (55% female, 45% male). The determinants of personal growth on happiness that emerged were life satisfaction, positive thinking about oneself, growth mindsets, opportunities for learning, perceived power, personal meaning, and positive engagement. All studies showed an increase in happiness caused by personal growth determinants such as personal growth, life satisfaction, positive thinking about oneself, growth mindsets, opportunities for learning, perceived power, personal meaning, and positive engagement. Emotional intelligence, personal growth initiative, and life satisfaction showed an association with happiness ^[83][94]. Growth mindsets led to stronger beliefs in the changeable nature of happiness and were found associated with greater well-being and greater relationship satisfaction ^[84][32]. Perceived power was positively related to happiness ^[85][95]. Other studies examined the association between meaning, positive engagement in happiness showed that meaning and engagement were positively associated with happiness ^[86][31]. Orientation to pleasure, meaning, and engagement (dimension-centered approach) was positively associated with happiness ^[87][96]. Rumination inducing messages led to less happiness, whereas hope-inducing messages led to greater happiness ^[88][97]. Higher personal mastery and positive health behaviors were positively correlated with happiness ^[89][98]. Higher resilience was associated with greater joviality and happiness ^[90][99].

3.3. Economic Growth and Happiness

A total of five studies examined the effects of economic growth on happiness. These studies employed the following happiness measures: The age range of the participants across these studies was between 15 and 91 years, and the participants were from 32 cultures across 6 continents and 100 countries that included Asia, Africa, America, China, Indonesia, National Survey, Pakistan, Philippines, and Thailand. Both female and male participants were present in these studies, the majority being female (52% female, 48% male). Three studies showed an increase in happiness caused by the determinants of economic growth that included increased economic growth, socio-economic status, and fiscal decentralization across economically diverse communities. Two studies showed a negative impact on happiness caused by the determinants of economic growth that included less economic freedom, economic scarcity, the earnings of others, unemployment, and economic disparity across communities. The determinants of economic growth on happiness, such as socioeconomic status, and fiscal decentralization increased happiness. Less economic freedom, economic scarcity, the earnings of others, unemployment, and economic disparity were determinants of economic growth that harmed happiness, as reported in the three studies referenced below. Subjective socioeconomic status and coming from a higher-income country positively correlated with happiness ^[91][100]. Rapid economic growth and rises in the price of housing led to greater happiness in older people than the youth ^[92][101]. Income did not affect the level of happiness of those who lived in either urban or rural areas ^[93][102]. Fiscal decentralization (improved capacity of districts to deliver public services) significantly increased citizen happiness ^[94][103]. Other studies looked at the impact of economic scarcity on happiness. Individuals with unemployment and low health status reported lower happiness ^[95][104]. A study found less economic freedom was negatively associated with happiness ^[96][105]. These studies show that social comparison rather than absolute earnings or economic status has a great influence on the assessment of happiness. Collectively, these studies show the impact of one’s economic status on happiness, whereas lower status has a greater negative impact on happiness.

4. Harmony and Happiness

Harmony emerged as a happiness determinants category, where 76 studies examined the determinants of harmony on happiness. For precision and simplicity, the harmony determinants were categorized under social, family, culture, and environment determinants based on the patterns that emerged across the studies.

4.1. Social Harmony and Happiness

A total of 12 studies examined the effects of social relationships on happiness and found a positive relationship between them. The age range of the participants across these studies was between 16 and 79 years, and the participants were from 34 countries including Asia, the Americas, Spain, Canada, Germany, South Africa, Slovak, Uganda, the UK, and the USA. Both female and male participants were present in these studies, the majority being female (54% female, 46% male). In total, 11 studies showed an increase in happiness caused by the determinants of social context that included prosocial behavior, social relations, life balance, leisure, social support, sense of community, socializing, developing positive thinking about social groups, nurturing social relationships, and social context. One study showed no significant relationship between social support and happiness. Study showed that individual-level happiness was determined by social context, i.e., age, education, employer status, and health ^[97][106]. Prosocial actions (acts of kindness towards others) led to greater increase in happiness than self-focused actions and neutral behaviors ^[98][107]. Prosocial spending was consistently associated with greater happiness ^[99][108]. Influence, social relations, life balance, optimism, work, and leisure were all positively associated with happiness levels ^[12][15]. Social relations, higher social support, and a sense of community, even online (Facebook), contributed to decreased loneliness and increased happiness ^{[100][101][102]}[109,110,111]. School belonging mediated the association between social and academic competence and students’ concurrent happiness ^[103][112]. However, a study found no significant relationship between social support and happiness ^[104][113]. These studies collectively show that a sense of belonging, good social relations, and support are important determinants of happiness. Other studies examined the effect of social activities on happiness. Training programs in happiness that centered on fundamentals such as keeping busy, spending more time socializing, developing positive thinking, and working on a healthy personality demonstrated significant happiness increase over a control group receiving summary instruction in the program ^[105][30]. Activities endorsed by happiness seekers included nurturing social relationships, practicing acts of kindness, pursuing goals, practicing religion and/or spirituality, using strategies to cope with stress or adversity, avoiding overthinking and social comparison, practicing meditation, goal evaluation and tracking, savoring the moment, gratitude journaling, thinking optimistically, remembering happy days, and strengthening social relationships ^[10][13].

4.2. Family Harmony and Happiness

A total of 33 studies examined the effects of harmony in the family on happiness and found a positive relationship between them. The age range of participants across these studies was between 3 and 96 years, and the participants were from Africa, China, Egypt, Europe, the Far East, Iceland, India, Iran, Japan, Korea, New Zealand, Nicaragua, Pakistan, Portugal, South Africa, South America, Spain, the UK, and the USA. These studies used both female and male participants, where the majority were female (65% female, 35% male). A total of 29 studies showed an increase in happiness caused by the determinants of family, which included family support, family communication, good connections with family, emotional support, home-dwelling elders, time spent with parents, positive mothering, positive marital relationship, entering cohabitation, perceived help from spouse, gender levels, women’s self-esteem, quality of experience in wife’s role, pregnant women, work-family conciliation, higher resilience, and women with higher affective intensity. Studies that revolved around family communication, emotional support, and family social support, showed that improved subjective happiness led to family happiness ^{[106][107][108][109]}[114,115,116,117]. Happiness was found to be positively associated with good connections with family and friends, school, regular exercise, and meals with family ^[110][118]. Family social support, i.e., cohesion, expressiveness, and conflict, showed a positive association with happiness ^[111][119]. Family communication, family well-being, and gratitude intervention improved family happiness ^[106][112][114,120]. Similarly, family communication, emotional support, and family social support were found leading to improved subjective happiness and family happiness ^[107][115]. Some studies examined the relationship between elders and happiness. Highly successful and home-dwelling elders demonstrated significantly higher happiness ^[113][121]. Emotional support from parents together with time spent with parents had the largest positive influence on happiness ^[114][122]. Higher perceptions of work–family conciliation predicted higher happiness ^[115][123]. Positive mothering led to increased joy and pleasure ^[116][124] and emotional deregulation ^[106][114]. Several studies examined marital relationships and happiness. Studies found that success in dealing with marriage contributed to happiness ^[77][29]; and that a positive relationship existed between marriage and happiness ^[117][125]. Higher happiness was found in a balanced marriage ^[118][126]; while fewer difficulties in a marital relationship status positively related to happiness and showed that perceived help from the spouse increased partners’ happiness ^[119][127]. A more balanced marriage with intra-couple education (both husband and wife are well-educated) demonstrated higher happiness ^[118][126]. Unhappily married couples showed a deficit in problem-solving, in more unresolved problems, less involvement with one another, and less shared sexuality ^[120][128]. Less happiness in marriage was caused by viewing explicit sexual movies ^[121][129]. Other forms of close, intimate relationships also contributed to happiness. Studies found that entering cohabitation is as beneficial as entering marriage and contributed to a peoples’ happiness ^[16][122][19,130]. Moreover, successful marital and parental relations were also positively associated with happiness. Therapist contact programs improved marital happiness ^[123][131]. In a study that was conducted in the USA with a racial/ethnic composition of the total enrolled sample that included both mothers and children, with 18% African American/Black, 79% Latino/Hispanic, and 1% of mixed racial/ethnic background, found that children of mothers living with HIV who underwent the Teaching, Raising, and Communicating with Kids (TRACK) program, exhibited increase in happiness ^[124][132]. Some studies examined women’s gender role’s impact on happiness. Self-esteem, the number of roles a woman occupied (e.g., paid worker, wife, mother), family income, being a paid worker, and quality of experience in a wife’s role were significantly, positively associated with pleasure ^[33][125][47,133]. Women declared a lower level of happiness compared to men in post-socialist countries ^[126][134]. By contrast, women with higher affective intensity than men were as happy as men ^[67][79]. Women disagreeing with subservient gender attitudes reported higher happiness ^[5][127][8,135], and research showed males had higher happiness levels than females ^[128][136]. Most pregnant women, of maternal age (21–40), and with no smoking history independently correlated with higher happiness ^[14][17]. Women with more planned pregnancies, and who had more difficulty in deciding to terminate, experienced lower levels of happiness when it came to deciding about abortions ^[129][137]. Most of the women in poverty/victims of intimate partner violence (IPV) showed an optimistic outlook, and higher feelings of social support led to greater happiness ^[130][138]. Finally, studies showed a decrease in happiness caused by various determinants of the family such as unhappy couples, negative marital relationships, viewing explicit sexual movies, and women terminating pregnancies ^[131][132][139,140].

4.3. Cultural Harmony and Happiness

A total of seven studies examined the effects of culture on happiness. The age range of participants across these studies was between 11 and 90 years, and the participants were from Australia, Canada, China, Germany, Ghana/Sub-Saharan Africa, India, Japan, Malaysia, Netherlands, Rwanda, Taiwan, Thailand, Turkey, and the USA. Both female and male participants were used in these studies, where the majority were female (56% female, 44% male). Six studies showed an increase in happiness caused by the determinants of culture, ethnicity, indigenous culture, multiculturalism, segregation, self-identification, and ethnic identification. Only one study showed a decrease in happiness caused by the determinants of culture. All studies showed an increase in happiness caused by the determinants of culture that included culture, ethnicity, religion, spirituality indigenous culture, multiculturalism, segregation, self-identification, ethnic identification, faith, forgiveness, religious attendance, tolerance, and spirituality. Studies found that the characterization of a happy person differed at a cultural level, ^[9][133][12,141], and found culture and polymorphism interacted to influence the perception of happiness. Some studies examined the role of indigenous culture on happiness. Indigenous Australians in remote areas reported higher levels of happiness ^[134][142]. Mountain indigenous peoples, females, the elderly, and those who were healthier, wealthier, highly educated, with western beliefs, who received medical benefits, and were without housing problems or financial difficulties were more likely to be happy ^[135][143]. Other researchers reported higher levels of happiness among indigenous people ^[134][135][142,143]. Other studies examined the role of identity, multiculturism, and segregation on happiness. National identification, ethnic identification, self-identification, strict identity duality, perceived acceptance, and feeling at home were significantly positively associated with happiness ^[136][144]. A positive relationship was found between perceived school multiculturalism and subjective happiness ^[137][145]. Decreased segregation was associated with a reduction in happiness among Black populations ^[138][146].

4.4. Religious Harmony and Happiness

A total of 12 studies examined the effects of religion on happiness. The age range of the participants across these studies was between 11 and 90 years, and the participants were from Australia, Canada, China, Germany, Ghana/Sub-Saharan Africa, India, Japan, Malaysia, Netherlands, Rwanda, Taiwan, Thailand, Turkey, and the USA. Both female and male participants were used in these studies, where the majority were female (56% female, 44% male). A total of 10 studies showed an increase in happiness caused by the determinants of religion, faith, forgiveness, religious attendance, tolerance, and spirituality. Two studies showed a decrease in happiness caused by spiritual struggles. Some studies examined the role of religious faith and forgiveness on happiness. The relationship between lifetime trauma and happiness was fully moderated for people who experienced a religious transformation ^[139][147]. A significant positive contribution of forgiveness (self, others, situation) was found to lead towards greater happiness ^[140][148]. Personal happiness was predicted by active religious involvement and regular attendance to religious services ^[141][142][149,150]. Religious attendance and religiosity were significant positive predictors of happiness ^[143][151]. Synagogue attendance, prayer and religious attendance were associated with greater happiness ^[144][152]. Happiness positively correlated with the characteristics of tolerance, helpfulness, beliefs, spirituality, responsibility, purposefulness, worthiness, trust, and reliability ^[145][153]. Religiousness positively affected with happiness ^[146][154]. Practicing Islamic-based gratitude exercises (associating blessings with Allah) raised participants’ happiness levels ^[147][155]. Subjective happiness was positively correlated with non-organized religious activity and intrinsic religiosity ^[148][156]. Other studies examined the role of spiritual struggles and forgiveness on happiness. More spiritual struggles were associated with less happiness ^[149][157]. Specifically, all five types of the religious and spiritual struggles assessed (divine, demonic, interpersonal, moral, and ultimate meaning) correlated significantly negatively with happiness ^[150][158].

4.5. Environmental Harmony and Happiness

A total of 10 studies examined the effects of the environment on happiness. The age range of participants across these studies was between 18 and 93 years, and the participants were from Australia, Taiwan, the UK, and the USA. Both female and male participants were used in these studies, where the majority were female (60% female, 40% male). Nine studies showed an increase in happiness caused by the determinants of environment such as ecology, aesthetic neighborhoods, park visitation, green environment, green space, more water, better air quality, quiet neighborhoods, dog ownership, horticulture therapy, and increased environmentally friendly fruit and vegetable consumption. One study showed a decrease in happiness caused by the determinant of environment that included disaster, whereas one study showed a decrease in happiness. Living in urban vs. rural areas was associated with greater happiness ^[53][65]. Park visitation and greater diversity of park activities were found to stimulate happiness ^[151][152][159,160]. Neighborhoods with higher levels of aesthetics, more water, green space, and higher perceived safety were associated with greater happiness ^[8][11]. Better air quality/less pollution and quietness in the neighborhood, a higher level of ecological diversity derived from a green environment, diversity of species, and perceived naturalness enhanced happiness ^[153][161]. Horticulture therapy that included plant cultivation and plant-related material application significantly improved happiness ^[154][162]. Whereas these studies show the relationship between harmony with the local environment and happiness, other studies show the relationship between the foreign environment and happiness. A study showed that travel created short-term happiness through emotional and relational experiences ^[155][163]. Harmony with the environment also pertained to environmentally friendly food consumption. Increased fruit and vegetable consumption was predictive of increased happiness ^[91][156][100,164]. Dog ownership increased happiness ^[157][165]. On the negative side, a study showed that environmental disasters significantly decreased happiness ^[107][115]. Age, leisure activity engagement, and the earnings of others in the neighborhood were negatively associated with happiness ^[158][159][166,167].