Over the past decade, the diffusion and use of the internet has grown rapidly, especially among adolescents and young adults [1]. From a developmental perspective, young adults may face important challenges in relationships with their family, peers, and society, the outcomes of which can be mediated and moderated by their psychological and biological characteristics [2]. On the one hand, youths can benefit from the digital revolution, which allows them to instantly search for information and communicate with others around the world [3]. In addition, there is evidence that mobile health smartphone apps can prevent diseases and improve the quality of life in young people [4]. However, on the other hand, scientific literature indicates that young adults are at higher risk of using the internet in a maladaptive way [5,6], to the point of developing internet addiction (IA) symptoms [7]. However, to date, official diagnostic criteria have not been identified [8]; although, the revision of the fifth edition of the Diagnostic and Statistical Manual of Mental Disorders (DSM-5) [9] has included an internet-related condition—the internet gaming disorder in Section III. This condition has been defined as a behavioral addiction that eventually leads to loss of control over internet gaming and functional impairment [10]. Some authors have defined IA as uncontrollable and obsessive-compulsive use of the internet, preoccupations regarding computer or technological devices, inability to control their use, poor time management, craving, and interpersonal problems [11-14]. Epidemiological studies have shown a prevalence of IA ranging from 6% to 35% among young adults [15,16] and from 6% to 21% among adolescents [17]. Although the highest prevalence has been reported among males [18], recent evidence has underlined an increase among females, with no significant gender difference [19]. The Developmental Psychopathology theoretical framework [20] offers a valid model to conceptualize clinical and subclinical psychological difficulties in young adulthood (such as IA) as it considers the development as a result of mutual influences between individual inherited genetic vulnerabilities and the quality of social experiences [21,22], particularly within the family context [23].

Genes involved in dopaminergic and serotoninergic systems are the candidate genes most frequently associated with IA [24]. Dopamine (DA) is an important monoamine that regulates various neural mechanisms, such as cognitive memory and emotional activity [25], and represents the final pathway of the reward system [26]. The availability of DA is regulated primarily by its transporter (DAT), which recaps dopamine at the level of nerve synapses [27]. The 3' untranslated region of the DAT1 gene contains a variable number of tandem repetitions (VNTRs) of 40 polymorphic base pairs, and the most frequent polymorphisms are 9 or 10 repetitions [28]. Neurobiological studies have shown a dysregulated dopamine transmission associated with IA [29,30], with a decreased level of expression of DAT in the striatum [31]. Another gene crucial to the dopaminergic system is the dopamine D4 receptor gene (DRD4), which is involved in a wide range of behavioral processes related to addiction, such as attention, motivation, and emotion [32]. This gene has a polymorphic 48-base pair VNTR in exon 3, and the most frequent polymorphisms are 4 (4R), 7 (7R), and 2 (2R) repeats [33]. Previous studies have underlined significant associations between the DRD4 4R genotype with increased attentiveness [34,35] that, in turn, has been associated with a higher risk of IA [36]. Besides the role played by dopamine, it has been evidenced that serotonin may also be implicated in IA. Serotonin (5-HT) plays an important role in feelings of well-being, happiness, anxiety, stress susceptibility [37], and depression [38]. The serotonin-transporter-linked polymorphic region (5-HTTLPR) is crucial for the fine regulation of 5-HT [39]. This functional polymorphism results in 2 common alleles, the short (S) and the long (L) [40], and studies by Sun et al [36] and Lee et al [41] have reported significant association between IA and the S/S genotype. Other studies involving allelic variants in monoamine oxidase A (MAO-A) activity have provided further support for the role played by serotonin and dopamine, given that this gene encodes an enzyme involved in the degradation of DA and 5-HT. The MAO-A gene has a 30-base pair repeat in the promoter region that affects transcriptional efficiency [42]. The high-activity variant (MAOA-H; 3.5 or 4 repeats) has shown significant association with impulsive personality traits [43] and tobacco and cannabis use [44], but other studies have reported conflicting results, with the low-activity variant (MAOA-L; 2, 3, or 5 repeats) associated with disordered gambling [45,46]. However, considering the complexity of IA, to date there is a dearth of studies that have considered possible interactions between genetic vulnerability and the risk provided by the social environment [47], although recent evidence has highlighted the genetic moderation on the impact of the environment on other disorders and behaviors related to addiction [48].

In this field, the quality of family functioning has been suggested to be one of the main environmental risk factors for the onset and maintenance of IA among young adults [49,50]. In particular, many studies have shown that the lack of parental emotional support and connectedness, and the poor quality of the relationships with parents, have an important influence on the risk taking and addictive behaviors of their offspring [51-53], including IA [54-56]. Moreover, it has been posited that the poor quality of family functioning is associated with a wide range of psychopathological problems [57-59], which in turn may lead to a higher risk for the onset of IA [60].

International research has widely demonstrated the comorbidity between IA and other psychological difficulties, both in terms of psychopathological symptoms and personality disorder [61,62]. Although several studies have shown that this relationship may be reciprocal and bidirectional [63,64], recent evidence has suggested that IA can be considered as the result of other psychopathological problems [20], resulting in a strategy to cope with psychological discomfort [2] and physical discomfort [65]. Depression and anxiety symptoms represent the 2 psychopathological areas most frequently associated with IA, both in clinical samples [66] and in the general population [67,68]. Moreover, significant associations with cluster C personality features have been evidenced [69], especially with traits of avoidant personality disorder [70].

From a biopsychosocial perspective [71], this exploratory study aims to evaluate the role played by biological, psychological, and social factors that may contribute to the etiology of IA in a sample of 104 young adults of the general population.

We hypothesized the presence of the following: (1) significant associations between IA and genetic polymorphisms in MAO-A (ie, MAOA-H genotype), 5-HTTPR (ie, S/S genotype), DRD4 (ie, 4R/4R genotype), and DAT1 (ie, 9-repeat genotype) genes, based on previous studies that have underlined the key role of these genes in the biochemistry of addiction disorder [45,72,73], including IA [36,41,74,75]; (2) significant differences in the quality of family functioning of young adults and psychological profiles (ie, depression and anxiety problems, avoidant personality problems) between youths with IA and the control group, as suggested by previous studies that have shown significant association between IA both with a poor quality of family functioning [49,50] and with psychopathological symptoms, especially in the areas of depression and anxiety symptoms [76,77] and traits of avoidant personality disorder [78]; (3) a mediation role played by the psychological profile of young adults on the relationship between family functioning and IA, based on studies that have shown a predictive effect of the quality of family functioning on the psychopathological profile of young adults [57,58] that, in turn, has been suggested to be a significant predictor of IA [70]; and (4) the moderator role played by MAO-A, 5-HTTPR, DRD4, and DAT1 genotypes on the relationship between family functioning and IA, as suggested by recent evidence that has underlined that individual genotype play an important role in moderating individual sensitivity to environmental events [79,80] and to the development of psychopathology related to addiction [48].

Over a period of 1 year, 150 young adults (65/150, 43.3% boys; 86/150, 56.7% girls) aged between 19 and 23 years were recruited for this study, through the collaboration of universities in the center-south of Italy. All youths signed informed consent, in which the study was illustrated in detail. To examine the youths’ genotype of the MAO-A, DAT1, 5-HTTPR, and DRD4 genes, we collected biological materials from human buccal swabs (the procedure is described below). In addition, the youths who agreed to participate in this study were administered self-report questionnaires (described below). This study was approved by the Ethical Committee of the Department of Dynamic and Clinical Psychology at Sapienza University of Rome, in accordance with the Declaration of Helsinki.

Youths were assessed through buccal swabs (Isohelix Swab Pack) by a group of psychologists, specifically trained for the purposes of the study. Subjects were aware of not having to eat (including chewing gum, candy, etc), drink (except water), smoke, and brush their teeth for at least 1 hour before sampling. Epithelial cell samples were carefully collected through buccal swabs. The biological samplings were transported, slightly chilled by Normative ice (+4°C), to the laboratories of the co-author, EP, for further processing. After buccal swabs were gathered, young adults filled self-report questionnaires (described below).

In this study, from the total sample of 150 youths, 16.6% (25/150) of youths who did not complete the assessment procedure and 12.6% (19/150) of youths for whom it was not possible to collect biological samples were excluded. The final sample consisted of 104 young adults (mean age 21.87, SD 2.29 years; 50/104, 48.1% males). All youths recruited were university students. Most of the youths recruited for the study lived in families (101/104, 97.1%). Of the parents, 88.5% (92/104) were married or cohabiting, whereas 11.5% (12/104) were separated. On the basis of the IAT cut-off for Italian validation (≥40) [82], for aims 1 and 2 of this study, the total sample was divided into 2 subgroups: (1) IA group: comprising youths who reported excessive internet use (35/104, 33.6%) and (2) control group: comprising youths in complete control of their internet use (69/104, 66.4%).

The allele and genotype frequencies of the DRD4, MAO-A, 5-HTTPR, and DAT1 genes among males (Table 1) and females (Table 2) are reported in this study.

Genotypes were distributed according to the Hardy-Weinberg equilibrium among male (DRD4: χ²₁=2.1, P=.14; 5HTTPR: χ²₁=.07, P=.78; DAT1: χ²₁=1.2, P=.26) and female (DRD4: χ²₁=.04, P=.83; 5HTTPR: χ²₁=.1, P=.65; DAT1: χ²₁=.4, P=.48) youths, with the exception of MAO-A (overall: χ²₃=7.8, P=.05; females: χ²₃=7.1, P=.007).

To verify the possible association between IA and genetic polymorphisms in MAO-A, 5-HTTPR, DRD4, and DAT1 genes among male and female youths, sex-stratified logistic-regression models were conducted. If the CI crosses 1, the difference between genotype groups can be considered not significant. As shown in Tables 3 and 4, no significant associations were found between IA and any considered genotypes, both among males and females (all CIs crossed 1).

To verify possible differences between the 2 groups in family functioning and the psychological profile of young adults (ie, depression, anxiety, and avoidant personality problems), an independent samples t test was conducted. Results showed significant differences between youths with IA and their peers in the control group in the scores of the FAD AR subscale scores and ASR scores in all psychopathological areas (Table 5).

On the basis of the previous analyses, we verified the possible simple and multiple mediation effects of depressive and anxiety symptoms and avoidant personality traits on the relationship between FAD AR and IA. Mediation analyses were performed using the SPSS Macro PROCESS [91], which provides coefficient estimates for the total, direct, and indirect effects of variables using ordinary least squares regression. In particular, Model 4 [91] was used, which allows us to estimate the indirect effects within a 95% CI. The indirect effect can be considered statistically significant if the CI does not include zero. In our first simple mediation model, we tested whether youth’s depressive symptoms mediated the effect of FAD AR on youth’s IA. Results showed that FAD AR was significantly associated with youth depressive symptoms (B=0.29; P=.002), which, in turn, was positively related to the youth’s scores on IAT (B=3.33; P<.001). Moreover, considering the relationship between FAD AR and IAT, both direct (B=1.90; P=.04) and total effect (B=2.90; P=.002) were significant (Figure 1).

With regard to the indirect effect, the bootstrap CI showed that the indirect path via depression was statistically significant (Table 6).

We then tested whether the anxiety symptoms of young adults mediated the relationships between the FAD AR and IA. The results showed that the indirect effect of FAD AR through anxiety symptoms of young adults was not significant (Table 6). However, both direct (B=2.50; P=.008) and total effect (B=2.90; P=.002) of FAD AR on IAT were significant. Moreover, anxiety problems of young adults were significantly associated with their scores on IAT (B=2.61; P=.005), but the relationship between FAD AR and depression in young adults was not significant (P=.12; Figure 2).

In relation to the possible mediation role played by avoidant personality traits of young adults on the relationship between FAD AR and IA, results showed that FAD AR was significantly related to their avoidant personality traits (B=0.32; P<.001) that, in turn, was significantly associated with the IA of young adults (B=3.38; P<.001; Figure 3).

The indirect path via avoided personality traits of young adults was also statistically significant (Table 6).

Finally, results of the multiple mediation analyses showed a significant total indirect effect (B=1.24; 95% bootstrapped CI 0.28-2.38), implying that the changes in ASR depression, anxiety, and avoidant personality collectively mediated the relationship between FAD AR and IA. However, the specific indirect effects of the 3 mediators were not significant (CI included zero; Table 7).

Finally, we evaluated the possible moderating role played by the genetic polymorphisms in young adults in MAO-A, 5HTTPR, DRD4, and DAT1 genes on the relationship between FAD AR and their scores on IAT. Moderation analyses were conducted using the method outlined by Hayes [91] (Model 1). The results showed that the relationship between FAD AR and IA was moderated by the genotype of the 5‐HTTLPR, MAO-A, and DAT1 genes in young adults. The genotype of the DRD4 gene in young adults did not moderate this relationship (P=.41). In particular, high scores on FAD AR were positively associated with high levels of IA, but only in the presence of the L/x genotype of the 5‐HTTLPR gene (B=.37; t₁₀₀=3.54, P<.001; S/S: P=.85; Figure 4).

In addition, the 9/x genotype of the DAT1 was significantly related with high scores on IAT (B=0.42; t₁₀₀=3.47, P<.001; 10/10: P=.56; Figure 5).

Finally, the results showed a significant interactive effect of the MAOA-H polymorphisms (B=0.44; t₁₀₀=3.55, P<.001; MAOA-L: P=.50; Figure 6).

This exploratory study aims to investigate the possible biological, psychological, and environmental risk factors associated with IA in a young adult population. On the basis of a biopsychosocial model [71] that considered IA as a result of a mutual influence between individual genotype, psychological profile, and social environment, this study aimed to verify the possible influence of genetic polymorphisms in MAO-A, 5HTTPR, DRD4, and DAT1 genes, considering the role played by the perception of young adults of their family functioning and their psychopathological difficulties. We considered the dopaminergic system (DAT and DRD4 genes), the serotonergic system (5-HTTPLR gene), and the monoamine metabolism pathway (MAO-A gene) for their central role in impulsive behaviors [92-94]. The neurotransmitter systems encoded by these genes are reportedly associated with alcoholism or other addictions [45,72,73,75,95-98]. Moreover, several studies have shown that these genetic polymorphisms contribute to the susceptibility of an individual to environmental influences [99-101] and can act as moderators in associations between the quality of the emotional environment provided by parents and the evolutionary outcomes of young adults [102].

Our first hypothesis was that the genotypes of the 5-HTTPR, DAT1, DRD4, and MAO-A genes differed significantly between the IA and non-IA groups. However, we found no significant association between IA and any genetic polymorphisms, in neither males nor females. Specifically, we found no association between the 5-HTTPR polymorphism and IA. Previous studies [36,41] have reported that the S/S genotype HTTLPR increased the risk of IA. Similarly, our study indicated a higher percentage of the S/S genotype in the IA group than in the control group, but there was no significant difference. Moreover, we found no association between the DAT1 genotype and IA. Previous studies have shown that the DAT1 polymorphism is associated with addiction behavior, such as alcoholism [103,104], substance use/abuse [105], and pathological gambling [106]. Some studies [72,107] have suggested that the DAT1 genotype 9/9 is associated with a higher risk for addictive disorders. However, the findings of our study are in line with the findings from studies by other researchers [74,75] that showed no significant association with internet-related addiction. Given these inconsistent findings, further research is needed to clarify whether the DAT1 gene may have an effect on IA. In addition, we found no differences in the DRD4 genotype between the IA and non-IA groups. The DRD4 4/4 polymorphism was shown to be a risk genotype for substance-related disorders [108]. Moreover, it was found to be correlated with decreased risk for inattention [34,35] and, consequently, with increased duration of internet use and a higher risk of IA [36]. However, other studies have underlined a higher risk for addictive disorders associated with the 7-repeat allele of DRD4 [109,110]. In our sample, we found a rarity of the 7-repeat allele (11/104, 10.5%) and, given that previous studies have shown that there was a statistically significant difference in allele distribution of the DRD4 gene between ethnicities [111], further evaluation is necessary for a better understanding of the possible influence of the DRD4 genotype on IA in specific ethnic groups. Finally, regarding the MAO-A gene, no significant association was found with IA. However, the literature has reported mixed findings with regard to its genotype that could be considered a higher risk to impulsive behaviors, with some studies showing a significant association with the MAOA low-activity variant [45,46], other studies with the MAOA high-activity variant [43,44], and some other studies that have not found any association [112,113].

Our second hypothesis was that the quality of family functioning perceived by youths and their psychological profiles differed significantly between the IA and non-IA groups. As expected, we found that youths in the IA group reported significantly higher scores (representative of a poorer family functioning) on the subscale of FAD AR and on all psychopathological areas considered (ie, depressive and anxiety symptoms and avoidant personality problems) than their peers in the control group. These findings are consistent with previous studies that have underlined that poor quality of family functioning is one of the main social-environmental risk factors for the onset and maintenance of addictive behaviors among young adult populations [51,52], including IA [49,50,114,115]. Families with poor AR are characterized by difficulties in showing their emotions [84]. In this field, some authors have suggested that, given that feelings are not expressed or tolerated, youths may be more susceptible to cope with their internal stress by themselves [116] and may use the internet excessively as a strategy to cope with negative emotions resulting from the interpersonal relationships with parents [117,118]. Moreover, it has been suggested that young adults who perceive a lack of supportive and intimate relationships with their parents are more likely to search for social support from virtual interactive experiences [119].

With regard to the possible association between IA and psychological difficulties of young adults, our results confirmed that youths of the IA groups showed higher scores on depressive, anxiety, and avoidant personality problems. These findings are in line with previous studies that have shown that young adults affected by IA also reported the presence of more severe psychopathological symptoms than their peers without IA [15], especially in the areas of depression [76,77], anxiety [120], and traits of avoidant personality disorder [70]. The nature of the association between IA and other symptomatic psychopathological areas is controversial. International scientific literature has suggested that psychopathological symptoms may cause or contribute to the onset of IA or, conversely, IA may cause or further exacerbate the course of other psychopathological symptoms [76,121,122]. However, recent longitudinal studies [123,124] have produced new evidence on the predictive role of psychological suffering on IA. In this field, some authors have suggested that youths may be at higher risk of developing IA in an attempt to cope with their psychological problems [2]. For example, a youth affected by depression could use the internet as a strategy to escape and cope with feelings of sadness and low self-esteem [122]; excessive use of the internet could be used by young people in an attempt to manage anticipatory anxiety associated with stressful situations and life events [66]; people with avoidant personalities are often socially inhibited and have difficulty communicating with others in face-to-face situations [125], and, thus, they may use web-based communication to seek interpersonal interactions in a way safer and easier for them [126].

Our third hypothesis was that the psychopathological symptoms of young adults might mediate the relationship between FAD AR and IA. International research has shown that poor quality of family functioning is a significant predictor of the psychopathological symptoms of young adults [2,127], which, in turn, has been reported as a significant predictor of IA [123,124], suggesting a possible mediation role. Our results showed that depressive symptoms and avoidant personality problems of young adults mediated the relationship between FAD AR and IA. Moreover, our results have confirmed previous studies on the significant positive association between the perception of a poor AR of families of young adults to their depressive problems [127,128] and avoidant personality traits [129]. However, anxiety symptoms in youths did not mediate the relationship between FAD AR and IA, although the direct effect on IA was confirmed and is in line with studies by Sepehrian and Lotf [130] and Razieh et al [131]. The results of multiple mediation analyses showed only a significant total indirect effect, indicating that depression and anxiety problems, and avoidant personality traits together significantly explained the association between youth’s family AR and IA. However, the specific effects of the 3 mediators were not significant. As evidenced by Hayes [91], these results are only apparently contradictory, and could be as a result of the presence of a correlation between the mediators and the small size of the sample. Although previous research has underlined that psychopathological symptoms of young adults mediate the relationships between environmental risk factors (eg, parental psychopathological symptoms, stressful life events) and IA [132,133], this is the first study to explore the possible role played by family functioning in these processes.

Finally, our fourth hypothesis was that genetic polymorphisms in MAO-A, 5HTTPR, DRD4, and DAT1 genes might moderate the relationship between family functioning and IA. Our results showed that the relationship between FAD AR and IA was moderated by the genotype of the 5‐HTTLPR, DAT1, and MAO-A genes in young adults. In particular, poor AR perceived by youths in their family functioning was associated with a high level of IA, but only in the presence of the L/x genotype of the 5‐HTTLPR gene, the MAOA-H genotype, and the 9/x genotype of the DAT1. These findings are in accordance with recent evidence from geneXenvironment (GxE) interaction studies, which have indicated that not all individuals are susceptible to environmental influences to the same extent [134]. In this regard, the individual genotype (genetic variations) is considered to play a crucial role in moderating individual sensitivity to environmental exposure [79] and to the development of psychopathology related to addiction [48]. In particular, although studies by Lee et al [41] and Sun et al [36] have reported a significant association between IA and the presence of 2 copies of the short allele (S/S) of the 5-HTTLPR, our results are in line with previous studies that have reported an interaction effect between the L allele with stressful life events and a poor quality of family relationship on the onset of psychopathological problems [109,135], including difficulties in the area of addiction [136,137]. Moreover, the presence of the MAOA-H genotype gene has been shown to be a significant moderator in the relationship between family environmental exposure and addiction-related problems [138,139], and individuals carrying the 9-repeat allele of the DAT1 gene are at higher risk of alcoholism [140] and other addictive problems [72,107]. However, to the best of our knowledge, this is the first study to explore the possible influence of GxE on IA.

This study has some limitations. First, the small sample size and the resulting limited statistical power that should be taken with caution in our preliminary findings, which should be confirmed by further studies with larger samples. Moreover, the cross-sectional nature of the study did not allow testing of causal links between the variables taken into account, which should be explored in subsequent longitudinal studies. Moreover, we used a self-report tool for the assessment of IA of young adults, their family functioning, and psychopathological difficulties. Although these tools have proven their worth in numerous studies, future studies should evaluate these variables through more robust methodologies, such as observational procedures or clinical interviews. Despite these limitations, this exploratory study is the first to focus on the possible moderation of genetic polymorphisms in MAO-A, 5-HTTPR, DRD4, and DAT1 genes on the relationship between the quality of family functioning and IA of young adults, reporting significant GxE effects. Overall, our findings add new evidence with regard to the biological, individual, and social risk factors associated with IA in the young adult population, which can be evaluated for the development of more targeted and effective intervention programs.