Data Collection and Sample
We recruited our sample using targeted advertising on Facebook and Reddit, two of the most popular social media platforms. This method of recruitment has increasingly been proving its value in providing an efficient and inexpensive way of recruiting large and diverse samples (Forgasz et al.,
2018; Ramo & Prochaska,
2012). We directed individuals responding to our ads to an online survey platform and offered potential participants a chance to win a $50 Visa gift card in a random draw as a participation incentive. This incentive was approved by the overseeing ethics committee as it was deemed to be an appropriate incentive given what was required of participants. We initially targeted Australian males and females equally with our ads, however, as commonly found with this form of recruitment, females were responding at higher rates than males (see Guo et al.,
2016; Marconi et al.,
2019; Smith,
2008). For this reason and the fact that we were interested in cross-gender comparisons, as the data collection progressed, we targeted males more heavily with our ads so we might obtain a gender ratio that more closely reflected general population proportions.
In total, 3,092 individuals initiated the survey (67% males). We reduced this number in our analytic sample in the following ways. Respondents who indicated that they had no intimate relationship in the prior year were excluded because they did not have the opportunity to engage in IPV and therefore did not meet selection criteria for the study (n = 142). Following this, we removed 826 respondents due to substantially incomplete survey responses and 33 individuals who reported non-binary forms of sex/gender. This brought the sample to 2,091 (1,373 males and 718 females).
As the current study aimed to understand predictors for the levels of IPV offenders’ specialization, the sample was further restricted to those who self-reported at least one IPV offense (n = 328). Because three individuals had quick survey completion times or otherwise had response patterns that indicated they had not read each question carefully or taken the survey seriously, we removed these responses to reduce measurement error. These procedures resulted in a final analysis sample of 325 respondents: 155 females and 170 males.
Descriptive statistics for the sample’s demographics are presented in Table
1. Of note is that the sexuality was diverse in the sample with 14.7% of males and 38.7% of females identifying as not solely heterosexual. Eleven males (6.5%) identified as bisexual, 8 (4.7%) as gay, 6 (3.2%) as questioning or unsure, 3 (1.8%) as pansexual, 3 (1.8%) as demisexual and 1 (0.6%) as queer. Forty-four females (27.7%) identified as bisexual, 13 (8.4%) as pansexual, 5 (3.2%) as queer, 5 (3.2%) as questioning or unsure, 3 (1.9%) as a lesbian and 2 (1.3%) as asexual.
Table 1
Sample demographics and descriptive statistics
Education |
Did not complete high school | 21 | 12.4 | 9 | 5.8 |
High school certificate | 19 | 11.2 | 26 | 16.8 |
Tafe or vocational certificate | 56 | 32.9 | 26 | 16.8 |
Undergraduate diploma | 10 | 5.9 | 9 | 5.8 |
Undergraduate degree | 36 | 21.2 | 48 | 31.0 |
Honours | 4 | 2.4 | 5 | 3.2 |
Post-graduate diploma or similar | 9 | 5.3 | 16 | 10.3 |
Post-graduate degree | 15 | 8.8 | 16 | 10.3 |
Australian Born | 135 | 79.4 | 127 | 81.9 |
Racial Minority | 26 | 15.3 | 34 | 21.9 |
Heterosexual | 145 | 85.3 | 95 | 61.3 |
Opposite Sex Partner | 153 | 90.0 | 143 | 92.3 |
Relationship Status |
Married | 108 | 63.5 | 80 | 51.6 |
Current Serious Relationship | 11 | 6.5 | 43 | 27.7 |
Past Serious Relationship | 31 | 18.2 | 16 | 10.3 |
Dating Relationship/s | 6 | 3.5 | 5 | 3.2 |
Intimate or sexual partner/s | 3 | 1.8 | 3 | 1.9 |
Multiple Relationship types | 11 | 6.5 | 8 | 5.2 |
| Mean (SD) | Range | Mean (SD) | Range |
Age | 45.50 (14.83) | 17 – 77 | 30.62 (12.59) | 18 – 68 |
While the diversity of sexuality could be considered high for the females in the sample, this is not an uncommon finding in contemporary survey research. For example, in an Australian longitudinal study of women’s health, between 35 to 40% of women aged 22 to 28 did not identify as exclusively heterosexual (Women’s Health Australia,
2019). It is also worth noting that the majority of respondents in the current study reported on opposite-sex relationships (90.0% males and 92.3% females).
Measures
This scale is an attitudinal measure of self-control and includes items such as, “I say inappropriate things” and “sometimes I can’t stop myself from doing something, even if I know it is wrong” as well as reverse coded items like “I refuse things that are bad for me”. Respondents were asked how much the 13 statements reflect how they typically are as a person. Response options were ranked from 1 = “not at all” to 5 = “very much”.
We should mention that the Brief Self-Control Scale is typically applied as a unidimensional measure for self-control as suggested by the authors of the scale (Tangney et al.,
2004); however, some have applied multidimensional operationalizations. A study looking at the dimensionality of the Brief Self-Control Scale found that two-dimensional measures did not substantially enhance predictive power, and the total score is a viable option for assessing self-control and for studying its relationship with outcome variables (Lindner et al.,
2015). For this reason, we followed the prior precedent within the literature and summed all 13 values after accounting for the reverse-coded items. This resulted in a single variable where higher scores indicate greater self-control.
Nine respondents skipped one or two questions on the scale. The mean value of their other responses on this scale was used to replace these values. The resulting scale had a fairly high level of internal consistency for both males (α = 0.838) and females (α = 0.798).
For our measure of intimate partner violence perpetration, we included all of the items from the physical assault subscale. Due to the growing understanding that intimate partner violence frequently includes non-physical forms of abuse, we also included items from the psychological aggression and sexual coercion subscales that were considered severe by Straus et al. (
1996). In total, sixteen items were used to measure the frequency of IPV perpetration.
Respondents were asked about behaviors that occurred during the past year on a 7-point frequency scale ranging from “not at all” to “more than 20 times”. Response categories that included an interval (e.g., 3–5 times) were recoded as the interval’s mean. Respondents who selected “more than 20 times” were coded conservatively as 20. As the questions were framed around a time frame (i.e., 12 months) rather than a specific relationship, respondents may have been reporting on a past partner, a current partner, or both. Overall, the IPV measure had reasonably high scale reliability for both males (α = 0.748) and females (α = 0.951).
We used twenty items in the non-IPV criminality measure; seven property offense items, five violent offense items, six drug offense items, and two traffic offense items. Fifteen items were not used because they included innocuous acts that would not generally be considered criminal or were about “considering” committing an offense but not actually doing so (e.g., “Considered leaving a restaurant without paying”). As mentioned earlier, these 15 excluded items were never intended for use in a final scale, but were placed into the survey for alternative reasons.
The 20-item scale had acceptable internal consistency (α = 0.821 for males and α = 0.841 for females). A full list of IPV and non-IPV criminality survey items are included in the Appendix Table
6. Descriptive statistics for the key variables are presented in Table
2.
Table 2
Descriptive statistics of the independent and dependent variables
Self-Control | Males | 41 | 36 | 41.18 | 9.45 | 17 – 65 |
Females | 39 | 47 | 39.06 | 8.53 | 14 – 58 |
IPV | Males | 2 | 1 | 6.42 | 12.47 | 1 – 79 |
Females | 3 | 1 | 13.72 | 38.62 | 1 – 284 |
Non-IPV Criminality | Males | 4 | 0 | 13.06 | 23.02 | 0 – 128 |
Females | 2 | 0 | 11.21 | 25.56 | 0 – 163 |
Violence | Males | 0 | 0 | 3.28 | 7.25 | 0 – 40 |
Females | 0 | 0 | 1.49 | 6.46 | 0 – 60 |
Property | Males | 0 | 0 | 1.86 | 4.64 | 0 – 33 |
Females | 0 | 0 | 2.81 | 9.34 | 0 – 77 |
Drugs | Males | 0 | 0 | 4.89 | 13.09 | 0 – 75 |
Females | 0 | 0 | 5.84 | 13.73 | 0 – 75 |
Traffic | Males | 1 | 0 | 3.04 | 6.42 | 0 – 35 |
Females | 0 | 0 | 1.07 | 3.00 | 0 – 22 |
Respondents were asked about their place of birth as either: Born in Australia, Foreign-born/ born overseas, I prefer not to answer, and Other. All respondents indicated that they were born in Australia or overseas, and for this reason the variable was coded dichotomously into 1 = “Australian born” and 0 = “not Australian born”.
Cultural background was measured with a multiple selection question that included options for Caucasian/white, Aboriginal or Torres Strait Islander, Asian, Pacific Islander, I prefer not to answer, and an option for other where respondents could specify a different cultural background. Caucasian/white was by far the most prominent response. Due to the small number of other responses, we recoded this measure into a dichotomous variable indicating whether the participant self-identified as a racial/ethnic minority with 1 = “racial minority” and 0 = “non-minority”. Those who indicated both (n = 19), were coded as a 1.
Sexuality was measured with a question where respondents could select multiple options. The response options were heterosexual (straight), gay, lesbian, bisexual, asexual, demisexual, pansexual, queer, questioning, or unsure, I prefer not to answer and an option where respondents could specify an identity that was not listed. Heterosexual had the highest selection frequency. This variable was recoded into a dichotomous measure of sexuality where 1 = “heterosexual” and 0 = “non-heterosexual”. Some respondents selected heterosexual and another answer category (n = 11). These respondents were coded as a 0.
Analytical Approach
The current project was primarily interested in assessing the predictive ability of IPV offenders’ level of self-control on the level of IPV specialization. Two measures of specialization were used. The first is a multilevel item response theory (IRT) measurement approach (Osgood & Schreck,
2007). This approach models IPV specialization as a latent variable. Individual-level explanatory variables, in this case self-control and the demographic control variables, can be used to predict specialization across individuals. A two-level model was used, in which offense categories (i.e., IPV, violent, property, drug, and traffic crime) were nested within individuals.
The multilevel IRT measurement approach was a good fit for the project as it has substantial advantages when using self-report data through the use of the IRT framework and it can be applied to research questions that include mutually exclusive categories of offenses (i.e., IPV and non-IPV). Unlike other measures of specialization (see Sullivan et al.,
2009), this approach defines specialization independent of the overall rate of offending and has the ability to examine a variety of IPV and non-IPV offenses in a way where the results are not confounded by differences in offense base rates (Osgood & Schreck,
2007) that occur when the most serious of offenses are committed less frequently, as is often the case with offense data (Osgood et al.,
2002).
The approach can be used with dichotomous (e.g., Sullivan et al.,
2009) or frequency/count measures (e.g., McGloin et al.,
2011) of offending. In the current study, frequency measures were used. For this reason, we followed the suggestions and/or precedent of the originators of this model (Osgood & Schreck,
2007; Schreck et al.,
2012) as well as other scholars who have applied this IRT framework specifically to questions about IPV specialization (Bouffard & Zedaker,
2016) by using Poisson regression within the IRT framework. The Poisson model properly accounts for the skewed nature of the offense data, which are counts of the number of occurrences of the different offense types and as such are not normally distributed.
Following Osgood and Schreck’s precedent, we included a Spec variable that accounts for the proportion of IPV offenses to non-IPV offenses. The Spec variable for IPV items was coded as the proportion of items not classified as IPV, while the non-IPV items were coded as the negative proportion of items that were classified as IPV. For this reason, the values of this variable sum to zero for each individual. In this case, there were five crime categories; one IPV and four non-IPV categories. When the data are nested at the first level by offense type, Spec equaled 0.80 for the IPV category and -0.20 for the four other offense categories.
Equation
1 illustrates the level-1 model. Here, B
0j is a latent measure that corresponds to overall offending. B
1j is another latent measure that corresponds to the specialization variable, which represents the degree of variance in IPV offense concentration in the sample. B
ijDij represents the intercepts for each offense category (
D) and accounts for the base rate of each category. These variables account for the fact that more serious offenses are generally committed with less frequency than are less serious offenses. When respondent
j endorses offense
i, then Y
ij is equal to the frequency of the offending in that crime category, and when that respondent does not endorse the offense, then Yij equals 0.
Level-1 Model for the Multilevel IRT Measurement Approach
$$\begin{array}{c}E\left({Y}_{ij}\left|{B}_{j}\right.\right)= {\lambda }_{ij}\\ log\left[{\lambda }_{ij}\right]={B}_{0j}+{B}_{1j}Spec+\sum_{i=2}^{I}{B}_{ij}{D}_{ij}\end{array}$$
(1)
Equation
2 shows the expanded form of the level-1 model for the male and female sample that includes the offending category variables used in the current study. The property crime offense category was excluded from the model as a reference category.
Level-1 Model for the Multilevel IRT Measurement Approach
$$\begin{array}{c}E\left({Offence}_{ij}\left|{B}_{j}\right.\right)={\lambda }_{ij}\\ log\left[{\lambda }_{ij}\right]={B}_{0j}+{B}_{1j}({Spec}_{ij})+{B}_{2j}({IPV}_{ij})+{B}_{3j}({Violence}_{ij})+{B}_{4j}({Drugs}_{ij})+{B}_{5j}({Traffic}_{ij})\end{array}$$
(2)
Equation
3 shows the level-2 model that was used. Each variable included in the first level becomes an outcome variable at the second level. Individual-level predictors can be included to predict these outcome variables. In this case, predictors were included for overall offending (B
0j) and specialization (B
1j). Variables were entered as uncentered in the second level as the focus was on the slopes rather than the intercept (Woltman et al.,
2012).
Level-2 Model for the Multilevel IRT Measurement Approach
$$\begin{array}{l}{B}_{0j}={\gamma }_{00}+{\gamma }_{01}\left(Age\right)+{\gamma }_{02}\left(Education\right)+{\gamma }_{03}\left(AustralianBorn\right)+{\gamma }_{04}\left(RacialMinority\right)+{\gamma }_{05}\left(Heterosexual\right)+{\gamma }_{06}\left(SelfControl\right)+{U}_{0j}\\ {B}_{1j}={\gamma }_{10}+{\gamma }_{11}\left(Age\right)+{{\gamma }_{12}\left(Education\right)+\gamma }_{13}\left(AustralianBorn\right)+{\gamma }_{14}\left(RacialMinority\right)+{\gamma }_{15}\left(Heterosexual\right)+{\gamma }_{16}\left(SelfControl\right)+{U}_{1j}\\ \begin{array}{c}{B}_{2j}={\gamma }_{20}\\ {B}_{3j}={\gamma }_{30}\\ \begin{array}{c}{B}_{4j}={\gamma }_{40}\\ {B}_{5j}={\gamma }_{50}\end{array}\end{array}\end{array}$$
(3)
The second measure of specialization used in the current study was the OSC. This is a measure of the proportion of offenses that are for a particular crime type, which in this case was the proportion of IPV offenses in an individual’s reported criminal history. This approach was selected due to its popularity in prior specialization literature and its simplicity, which permits easier application and interpretation. The OSC is calculated using the following formula (Eq.
4), where the frequency of IPV offenses is divided by the total number of offenses (both IPV and non-IPV). An OSC value of 1 indicates perfect specialization, while a low OSC value indicates generalization. In the current sample, the mean OSC was 0.44 (
SD = 0.35) for the males and 0.56 (
SD = 0.35) for the females.
Offense Specialization Coefficient Formula
$$OSC=\frac{IPV}{total}$$
(4)
The predictive ability of self-control on specialization as measured with the OSC was examined using a linear regression model that included self-control and the five demographic control variables.