Abstract
Intimate partner violence (IPV) and child abuse (CA) are two forms of family violence with shared qualities and risk factors, and are forms of violence that tend to overlap. Acceptability of violence in partner relationships is a known risk factor in IPV just as acceptability of parent–child aggression is a risk factor in CA. We hypothesized that these acceptability attitudes may be linked and represent the expression of a general, underlying nonspecific acceptance of violence in close family relationships. The sample involved 164 male IPV offenders participating in a batterer intervention program. Implicit measures, which assess constructs covertly to minimize response distortions, were administered to assess acceptability of partner violence against women and acceptability of parent–child aggression. To determine whether acceptability attitudes regarding both forms of violence were related to a higher order construct tapping general acceptance of family violence, Bayesian confirmatory factor analyses were conducted. Findings supported a hierarchical (bifactor) model with a general factor expressing a nonspecific acceptance of family violence, and two specific factors reflecting acceptability of violence in intimate partner and parent–child relationships, respectively. This hierarchical model supporting a general acceptance of violence in close family relationships can inform future research aiming to better understand the connections between IPV and CA.
Intimate partner violence (IPV) and child abuse (CA) are two expressions of family violence that share commonalities and risk factors (Capaldi, Kim, & Pears, 2009; Daro, Edleson, & Pinderhughes, 2004; Hamby & Grych, 2013). Either form of family violence also amplifies the likelihood of the other form occurring (Capaldi, Knoble, Shortt, & Kim, 2012; Graham-Bermann & Edleson, 2001; Margolin, Gordis, Medina, & Oliver, 2003; Rumm, Cummings, Krauss, Bell, & Rivara, 2000; Schumacher, Feldbau-Kohn, Slep, & Heyman, 2001), wherein the literature illustrates both tend to overlap as evident in the high rates of co-occurrence of IPV and CA (see Appel & Holden, 1998; Capaldi et al., 2009; Edleson, 1999; Hamby & Grych, 2013; Tolan, Gorman-Smith, & Henry, 2006, for reviews).
Despite being forms of family violence that are interconnected, research addressing IPV and CA has traditionally lacked integration (Banyard, 2013; Gelles, 2000; Hamby, 2014; Hamby & Grych, 2013; Tolan et al., 2006). This lack of integration not only compromises communication among researchers but also limits integration of knowledge relevant to identifying common issues in different types of violence in family relationships. Recently, a growing number of scholars are calling for greater collaboration and integration of research addressing different areas or subfields of family violence (Banyard, 2013; Capaldi et al., 2009; Gelles, 2000; Grych & Swan, 2012; Hamby, 2011, 2014; Hamby & Grych, 2013; Heise, 2011; Jewkes, 2014; MacMillan & Wathen, 2005; Tolan et al., 2006). Establishing new links between these research traditions clearly will take advantage of the potential overlap between different types of violence. Moreover, such a process will allow, as Heise (2011) pointed out, a much-needed cross-fertilization between these research communities, to move the field forward toward a more integrative understanding of risk and protective factors of family violence. Studying interconnections, and building bridges to work across different types of family violence, can advance not only our understanding of their causes but also our response to this violence in a more integrative manner (Banyard, 2013; Edleson, 2001; Gelles, 2000; Hamby, 2014; Hamby & Grych, 2013).
Attitudes of Acceptability of Violence
In this study, we aim to explore the link between attitudes of acceptability of violence in intimate partner and parent–child relationships, analyzing whether these attitudes are the expression of a general, underlying nonspecific acceptance of family violence. The intersection between IPV and CA supports the prospect of a common ground connecting these two types of family violence. However, research on attitudes of acceptability of violence in partner and parent–child relationships has typically followed different paths.
Perceiving aggression to be acceptable in interpersonal conflicts within the family is a recognized risk factor in the occurrence of IPV (Archer & Graham-Kevan, 2003; Fincham, Cui, Braithwaite, & Pasley, 2008; Gracia, Rodriguez, & Lila, 2015; Stith & Farley, 1993). Likewise, acceptability of physical aggression toward children has been linked to increased CA risk (Ateah & Durrant, 2005; Kelley, Grace, & Elliott, 1990; Milner, 2000; Rodriguez, Bower-Russa, & Harmon, 2011). However, scholarly attention has not explored whether attitudes of acceptability of violence in both partner and parent–child relationships are linked. As these two types of family violence are related, risk factors such as attitudes of acceptability of violence in intimate partner and parent–child relationships should also be linked. Therefore, these attitudes of acceptability are hypothesized to be linked as an expression of general acceptance of violence in close, family relationships.
The link between these two attitudes of acceptability would be consistent with social information processing (SIP) models of aggressive behavior (Crick & Dodge, 1994; Huesmann, 1988; Milner, 2000). The SIP model implies that preexisting cognitive schemas, including the acceptability of violence in close relationships (e.g., approval of parent–child aggression, favorable attitudes toward violence against partners), will influence how an individual interprets and ultimately responds to conflictual situations (e.g., discipline strategies with children or conflict resolution with partners), thereby increasing the risk of aggressive responses. Acts of aggression may arise impulsively (Berkowitz, 1983, 1989) but can then be justified by the aggressor, as has been observed in abusive parents (Dietrich, Berkowitz, Kadushin, & McGloin, 1990), further contributing to development of attitudes approving of family violence. Thus, SIP theory would support that a broad, nonspecific acceptance of violence in close relationships could be a shared construct connecting more specific attitudes of acceptability of violence in intimate partner and parent–child relationships.
The Present Study
In this article, we aim to determine whether attitudes accepting of physical IPV toward women and accepting of physical aggression toward children are related to a higher order construct tapping a general acceptance of family violence. Toward this end, this study utilized implicit measures of attitudes of acceptability of violence with a sample of male perpetrators of IPV. Research on IPV and CA shares similar methodological challenges. Both tend to occur in the privacy of the home, and thus, measurements often have to rely on self-reports that assess sensitive issues regarding family violence. Particularly among offenders, respondents may fear negative consequences for their honesty, thereby producing distorted or false information (Bennett, Sullivan, & Lewis, 2006; Eckhardt, Samper, Suhr, & Holtzworth-Munroe, 2012; Gracia et al., 2015; Hamby & Grych, 2013; Rodriguez et al., 2011). In contrast, analog tasks can employ implicit means to assess constructs, engage less conscious processing of a response, minimize response bias, and serve as important complements to self-reports to advance theory (DeGarmo, Reid, & Knutson, 2006; Fazio & Olson, 2003; Rodriguez et al., 2011; Wilson, Lindsey, & Schooler, 2000). We, thus, utilized analog tasks to determine whether acceptability attitudes regarding physical violence among intimate partner and parent–child relationships are linked to a higher order construct tapping general acceptance of family violence. We conducted a factor analytic study in which we assessed a number of competing solutions to determine the best fitting model.
Method
Participants
The sample consisted of 164 males convicted for IPV against their female partners or ex-partners, eligible to participate in a community-based batterer intervention program in Spain. Offenders had been sentenced to less than 2 years in prison with no previous criminal record of IPV offenses and received a suspended sentence conditional on attendance in the intervention program.
Lifetime prevalence data for IPV against women in Spain is, according to several sources, around 13% (European Union Agency for Fundamental Rights, 2014; Ministerio de Sanidad, Servicios Sociales e Igualdad, 2015; Vives-Cases et al., 2011). Interestingly, and despite lower gender equality compared with other European Union countries, Spain has the lowest IPV prevalence rate among the 28 European Union member states (European Union Agency for Fundamental Rights, 2014; Gracia & Merlo, 2016). Batterer intervention programs are typically a cognitive–behavioral intervention, and have become in Spain a generalized intervention resource to address the prevention of future cases of IPV, both for convicted offenders in prison (those with longer sentences and/or previous convictions for IPV) as well as for perpetrators with suspended sentences who can benefit from participating in a community-based intervention program (Carbajosa & Boira, 2013; Ferrer-Perez, Ferreiro-Basurto, Navarro-Guzmán, & Bosch-Fiol, 2016; Lila, 2013). Participants in this study belong to this last group and, in this regard, they cannot be considered the most violent group within the Spanish IPV perpetrator population.
For this sample, the mean age was 39.13 years (SD = 10.84 years). More than 73.5% indicated they had a child. The majority were separated or divorced (39.7%), with 35.4% single, 24.4% currently married or living with a partner, and 0.6% widowed. Nearly 22% were immigrants to Spain of whom about 9% were Latin American, 7% European (other than Spanish), 5% African, and less than 1% Asian. The average time living in Spain was around 10 years. Educational attainment was largely homogeneous (<10% reporting any college, 46.3% reporting only primary school), with 48.2% unemployed and with a median annual family household income between 6,000€ and 12,000€ (the mean income in Spain for 2016 was 26,000 €/year, so the sample could be characterized as low income).
Measures
The Partner Violence Acceptability Movie (PVAM) task (Gracia et al., 2015) is an implicit measure of acceptability of physical IPV toward women based on reaction time. Seven 90-s video clips from commercially available films dubbed in Spanish depicting scenes of physical partner violence toward women are presented in random order. Instructions indicate that participants should stop the video playback if and when they believe the interaction has become violent. Scores are computed from the time lapsed from the first point of aggressive physical contact in the scene until the respondent terminates the video clip. Greater response latency in identifying a scene as abusive is considered an indicator of the participants’ greater acceptability of physical IPV against women. This analog task was validated with samples of college students and IPV offenders, demonstrating adequate psychometric properties and providing evidence for both content relevance (correlations with self-reported approval in both samples) and criterion validity (i.e., the PVAM differentiated between male and female students, and between students and perpetrators samples; this analog task also differentiated between perpetrators before and after completing a batterer intervention program, more sensitively detecting change among offenders than self-report measures; Gracia et al., 2015). Cronbach’s alpha for the resulting five video clips was .76.
The Spanish Parent–Child Aggression Acceptability Movie Task (Spanish Parent-CAAM Task; Rodriguez, Gracia, & Lila, 2016a) is an implicit measure of acceptability of parental physical aggression toward children. This analog task is a Spanish adaptation of the original Parent–Child Aggression Acceptability Movie (PCAAM) Task (Rodriguez et al., 2011). The Spanish Parent-CAAM presents eight 90-s video clips from films dubbed in Spanish, including four scenes of physical discipline and four of physical abuse. Participants are again asked to stop the video if and when they consider the scene to have become abusive. Spanish Parent-CAAM scores are computed based on the difference between the time of first physical contact from the caregiver toward the child and when the respondent stops the video clip. Similar to PVAM, greater response latency to stopping the video suggests greater acceptability of aggression toward children. The Spanish Parent-CAAM task has demonstrated validity in previous studies. For example, the Spanish Parent-CAAM significantly related to self-reported acceptability of physical punishment of children, attributions of children’s intention, propensity to punish children and CA potential among college students (Rodriguez et al., 2016a); with IPV perpetrators, the Spanish Parent-CAAM also related to self-reported approval of parent–child aggression and CA risk (Rodriguez et al., 2016b). Cronbach’s alpha for the Spanish Parent-CAAM in this sample was .76.
Procedure
Penitentiary social services staff identified men convicted for IPV and ordered to a community-based batterer intervention program. IPV offenders participated voluntarily in a larger study of risk factors and batterer intervention program outcomes. Participants were informed that neither participation nor refusal would affect their legal situation. Those who agreed to participate completed a written consent form. Data for this study were collected during the regular intake data collection before the start of the intervention program. Administered with laptop computers, participants completed demographic information immediately followed by the PVAM and Spanish Parent-CAAM tasks. Videos were watched privately using headphones in a room where the researcher is available for assistance as needed. Data were coded by random identification numbers and responses were kept confidential. The university ethics committee approved the study.
Analytic Plan
Bayesian confirmatory factor analysis (CFA) was used to assess the latent structure of our data to determine whether attitudes of acceptability regarding violence among intimate partner and parent–child relationships were related to a higher order construct tapping a general acceptance of physical violence in close family relationships. Bayesian structural equation modeling (BSEM) is an emergent and promising CFA framework that offers several advantages over the classical maximum likelihood estimation (B. Muthén & Asparouhov, 2012). BSEM has proven to be more accurate in the recovery of model parameters with small sample sizes (Lee & Song, 2004), and it also performs well with skewed distributions of parameter estimates (B. Muthén & Asparouhov, 2012). Moreover, BSEM analyses allow testing complex latent structures (i.e., when factor cross-loadings are expected) imposing strong and informative prior distributions to structural parameters that are usually fixed to zero. Given the limited size of the sample in this study, and the advantages of the BSEM, we used this methodological approach to test the hypothesis of a general, underlying nonspecific acceptance of family violence.
To test the hypothesis, we conducted a factor analytic study in which we fitted and compared three models of different latent complexity (see Figure 1). The first model (Figure 1a) was a simple one-dimensional model in which all indicators (i.e., from both the PVAM and the Spanish Parent-CAAM) loaded onto a single factor, implying that acceptability of partner violence and acceptability of parent–child aggression could be explained by the same construct. In the second model (Figure 1b), PVAM and PCAAM indicators loaded onto two separate and correlated factors, implying that acceptability of partner violence and acceptability of parent–child aggression are two different constructs that could be related. The third model (Figure 1c) was a bifactor model (Holzinger & Swineford, 1937), also known as hierarchical or general factor model (Reise, Morizot, & Hays, 2007; Yung, Thissen, & McLeod, 1999), or nested factor model (Gustafsson & Balke, 1993). In a bifactor or hierarchical model, each indicator loads not only on one general dimension (i.e., the general acceptability of violence factor), usually comprised of all indicators included in the model, but also on a specific dimension (i.e., acceptability of partner violence and parent–child aggression factors), comprised of a subset of indicators (i.e., video clips). Relations among these factors are, however, assumed to be orthogonal with the common and unique variance of these factors conceptualized as separate latent dimensions (e.g., Chen, West, & Sousa, 2006; Gibbins, Toplak, Flora, Weiss, & Tannock, 2012; Patrick, Hicks, Nichol, & Krueger, 2007). Thus, the shared variance of acceptability of violence in partner and parent–child relationships indicators (i.e., PVAM and PCAAM clips) not explained by their specific factors is identified by a general factor (i.e., a general, nonspecific acceptance of violence in close family relationships).

Tested CFA models.
Models were estimated with the Markov Chain Monte Carlo (MCMC) algorithm, setting four chains and 20,000 iterations (the first 10,000 were discarded as burn-in period). Parameter estimates were determined through expected a posteriori (EAP) method. Posterior SD was also obtained to delimit the bounds of parameter estimates, in a similar manner to standard errors in the classical maximum likelihood CFA. Model convergence was assessed via proportional scale reduction (PSR) factor, with PSR values close enough to 1 for each parameter as an indicator of good convergence (Gelman, Carlin, Stern, & Rubin, 2013).
To evaluate model fit, we used the deviance information criterion (DIC; Spiegelhalter, Best, Carlin, & Van Der Linde, 2002). The DIC is a generalization of the Akaike information criteria (AIC) particularly convenient in Bayesian model selection problems. It is defined as the expectation of the model deviance plus a correction that depends on the estimated number of parameters in the model (Gelman et al., 2013; Spiegelhalter et al., 2002). DIC is an absolute fit index, meaning that models with lower values of DIC have better fit to the data. We also obtained Plummers’s (2008) correction for DIC (DICC), which penalizes model complexity more severely than the classical DIC.
All statistical analyses were performed with Mplus 7 (L. K. Muthén & B. O. Muthén, 2012). The prior distributions used in the parameter estimation were the default priors incorporated by the package (i.e., noninformative normal distribution for factorial loadings and intercepts, and noninformative inverse-gamma distribution for variances). Preliminary analysis indicated that residual correlations of the last two clips from the PVAM task were quite strong—violating the local independence assumption of some of the tested models—so these clips were removed from the CFA analyses.
Results
Model Fit
The fit indices for the three models are showed in Table 1. Results indicated a better fit for the bifactor or hierarchical model. The DIC progression was clear: Adding more dimensions to the model improved the fit to the data, supporting the hierarchical latent structure of the construct. The DICC values followed the same tendency, although the differences between the two-dimensional and the bifactor models were less pronounced than with the DIC. However, according to Spiegelhalter et al. (2002), a difference of two to six in DIC values between two models provides sufficient support for the model with the lower fit values. We, thus, focus on the bifactor model.
Model Fit Statistics for the Three Tested Models.
Note. DIC = deviance information criterion; pD = estimated number of parameters; DICc = corrected DIC; PSR = proportional scale reduction.
Bifactor Model
All indicators from the analog tasks loaded positively onto the general violence factor, with standardized loadings ranging from .20 to .83 (see Table 1/Figure 2). Both PVAM and PCAAM indicators presented similar loadings in the general factor, with higher parameter estimates, on average, in the general factor than in the specific ones. In particular, the second and first video clips of the PVAM and PCAAM scales, respectively, only loaded in the general violence factor. The general factor explained 59.74% of the total common variance. Posterior SD values were generally low, with values around .10. This indicated a fair degree of precision for the parameter estimates.

Bifactor model results.
Regarding the specific factors, the PVAM and PCAAM tasks were in general moderately related to their respective specific factors, with the exception of the aforementioned video clips. PVAM standardized loadings ranged between .24 and .54, whereas PCAAM loading values were between .29 and .61. The proportion of variance of the PVAM indicators attributable to its specific factor was 16.26%, whereas the general factor accounted for the 30.13% of the variance due to these indicators. Regarding the PCAAM indicators, the specific factor accounted for almost the same proportion of variance (16.26%) as the general factor (16.58%). The posterior SD of the specific factors loadings was slightly higher than in the general factor (suggesting a relatively lower precision for the parameter estimates in the specific factors as compared with the general one).
Discussion
Acceptability of violence in intimate partner relationships and acceptability of parent–child aggression are known risk factors associated with IPV and CA, respectively. Given the frequent co-occurrence of these two types of family violence, attitudes of acceptability of these forms of family violence were hypothesized to be linked. To this end, in the present study, we aimed to determine whether measures of acceptability of physical violence in intimate partner and parent–child relationships were related to a higher order construct tapping a general acceptance of family violence. To test this hypothesis, we conducted a factor analytic study, and results from the Bayesian CFA supported a model with a general factor expressing a nonspecific acceptance of family violence with two specific factors, reflecting acceptability of partner violence and acceptability of parent–child aggression, respectively.
According to our results, and supporting our hypothesis, a hierarchical (bifactor) model was the best fitting model in comparison with the alternatives (i.e., unidimensional and two correlated factors). In this model, scores of each indicator loaded directly onto the general factor, and most of them also on one of their specific factors. This bifactor model reflected, thus, both general and domain-specific factors. In our study, the general factor (i.e., the nonspecific acceptance of family violence) represents the common variance among all specific indicators (i.e., those tapping acceptability of IPV toward women as well as acceptability of physical aggression toward children) and the specific factors would represent the remaining separable variance (Gibbins et al., 2012). These specific factors are distinct and uncorrelated, and are not subsumed by the general factor, as the general factor is assumed to be independent of the specific domain factors (e.g., Chen et al., 2006; Gibbins et al., 2012; Gustafsson & Balke, 1993; Patrick et al., 2007).
Our results also emphasize the relevance of this general factor, as all indicators loaded on a dominant broad factor of acceptability of family violence, with seven out of 13 indicators having higher loadings on the general factor than on their respective specific factors. Furthermore, this general factor accounted for the largest proportion of the variance as it explained 59.74% of the total common variance. This general factor also explained a larger portion of the variance of the indicators tapping acceptability of partner violence (wherein 30% of the variance of the specific indicators accounted for the general factor vs. 16% accounted for the specific factor). Regarding the indicators addressing acceptability of violence in parent–child relationships, the proportion of the variance accounted for by the general factor was similar (even slightly higher) to that of the specific factor. These findings further support the idea of a broad underlying factor of acceptability of violence in close family relationships. This general factor identifying the overlap or communality in measures of acceptability of violence in different types of family relationships conveys the importance of recognizing a common ground when researching family violence.
As in other areas in family violence research, traditionally, the study of common elements such as the acceptability of family violence has been scant, with minimal integration of their respective literatures. The general acceptance of family violence supported by our data contributes to a bridge across two types of violence that are interconnected, acknowledging the overlap between IPV and CA. Our results support the view of a common ground linking attitudes of acceptability of violence in intimate partner and parent–child relationships, two risk factors of IPV and CA, respectively. This nonspecific acceptance of family violence could be interpreted, in this regard, as a potential general risk factor for different types of violence within the family.
The present study is a first exploratory step with some important research implications. Factors that influence the development of such family violence approval attitudes require greater study given that the observed overlap in IPV and CA approval may reflect deeper underlying commonalities. Clearly, further exploration of issues such as the evolution of attitudes approving of family violence, or the role of these attitudes in the intergenerational transmission of violence (both IPV and CA), could advance our understanding of key issues in family violence research. For example, in this study, we used a sample of adult male offenders convicted of IPV. IPV perpetrators are more likely to have been abuse victims or to have witnessed IPV earlier in their life; these may be linked to the development of attitudes of acceptability of both types of family violence, and the corresponding higher risk of perpetrating both IPV and CA (e.g., Capaldi et al., 2012; Copp, Giordano, Longmore, & Manning, 2016; Graham-Bermann & Edleson, 2001; Hamby, Finkelhor, Turner, & Ormrod, 2010; Lichter & McCloskey, 2004; Margolin et al., 2003; Schumacher et al., 2001). Attitudes have been considered a key component to explain the link between early experiences of violence within the family (e.g., witnessing violence among parents or being a victim of CA) and later perpetration of violence, such as CA and IPV (Copp et al., 2019; Flood & Pease, 2009; Markowitz, 2001). From a social learning perspective, a number of scholars have proposed that attitudes such of acceptability of family violence may reflect a socialization process, whereby violence is internalized as acceptable, and its use endorsed as an appropriate strategy to resolve relationship conflicts (Bandura, 1973; Copp et al., 2019; Flood & Pease, 2009; Lichter & McCloskey, 2004; Markowitz, 2001; Obeid, Chang, & Ginges, 2010; Simons, Lin, & Gordon, 1998). In this regard, future research would benefit from analyzing such attitudes among other high-risk samples such as abusive parents, as well as in samples where CA and IPV are co-occurring in the same household.
As for intervention implications, this study demonstrates that a general, nonspecific acceptance of family violence is linked to acceptability attitudes of IPV and CA. Acknowledging this broad, general attitude may contribute not only to advance our understanding of the causes of violence within the family but also to approach prevention and intervention initiatives from a more integrative and broader perspective (Banyard, 2013; Grych & Swan, 2012; Hamby, 2011; Hamby & Grych, 2013). If a general, nonspecific acceptance of family violence is a risk factor for both IPV and CA, these general attitudes should be addressed as part of a broader violence prevention and intervention agenda. Rates of different subtypes of family violence will continue to be a major problem as long as its use remains an acceptable or tolerable form of interaction in either parent–child or intimate partner relationships. Lay definitions of what constitutes IPV and CA are linked to the acceptability of violence of family relationships (Gracia, 2014; Gracia & Herrero, 2008a, 2008b; Heise, 2011). Thus, addressing general attitudes of acceptance of family violence in any form would represent part of the same agenda. As Hamby and Grych (2013) noted,
most of the identified causes of violence are general causes that produce many types of violence . . . From an intervention perspective, the many interconnections are good news, because they suggest that reductions in one type of violence can radiate outward to produce reductions in other forms of violence. (p. 108)
Finally, this study has strengths and limitations. Examining acceptability attitudes regarding two different types of family violence using implicit measures of attitudes as well as our analytic approach are among the strengths of this article. As for limitations, this study involved a high-risk sample of IPV male perpetrators, but the generalizability of this study to other lower or even higher risk samples may be limited. Regarding implications in terms of diversity, future research should replicate these findings across other high-risk samples and with samples of different ethnic, gender, and age compositions (e.g., abusive mothers and fathers, child victims of abuse, adolescents victims or perpetrators of different types of violence such as dating violence or bullying, female perpetrators of IPV). In this study, information about whether the IPV perpetrators had also been previously involved in cases of CA (as parents or as children) was not available, and as such, the potential influence of these factors in the development of attitudes of acceptability of family violence is unclear. From a more universal prevention standpoint, replication with a lower risk sample, and in other ethnic groups and cultural contexts, would clarify whether attitudes accepting of family violence are also present as a general underlying acceptability of family violence construct. Indeed, if in the general population, acceptability attitudes also reflect a general family violence acceptability construct, shifting attitudes in one domain of violence may spill over to mitigate acceptability of family violence in another domain. However, if such a general acceptability of family violence construct is not observed in lower risk samples (i.e., IPV acceptability is viewed as distinct from CA acceptability), then identifying what factors lead one to adopt a more general family violence acceptability perspective becomes more pressing. Clearly, future research would benefit from addressing these important research issues as we strive to reduce the public health consequences of family violence.
Footnotes
Declaration of Conflicting Interests
The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.
Funding
The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: Enrique Gracia was supported by the Spanish Ministry of Economy and Competitiveness (PSI2014-54561-P); Christina Rodriguez was supported by the University of Valencia’s program “VLC-CAMPUS Atracció de Talent” (Grant Nº UV-INV-EPC12-67851). Manuel Martín-Fernández was supported by the FPI program of the Spanish Ministry of Economy and Competitiveness (BES-2015-075576).
