Participants

Data for the current study were taken from the Mater Hospital and University of Queensland Study of Pregnancy (MUSP) in Australia [34], Baseline data were collected at the first antenatal visit to the Mater Public Hospital in Brisbane between 1981 and 1983 from 7223 consecutive women. Additional assessments were conducted when the study children were 6 months, 5 years, 14 years, 21 and 30 years old. The Mater Hospital and the University of Queensland Ethics committees approved this study. The present analysis uses data from the 21 and 30 year follow-up surveys. At these phases of the study, written informed consent was obtained from the participants. The sample was derived from 3271 and 2401 persons who participated in the 21 and 30-year follow-ups. A cohort of 1265 cases including 874 females and 391 males who participated at both phases and had a partner at the 21-year follow up comprised the study sample. Some 69.1% of the sample were females. The mean age at the 21-year follow up was 20.61 (SD ± 0.84) (Males = 20.61 ± 0.87; Females = 20.61 ± 0.83) and at the 30-year follow up was 30.28 (SD ± 1.12) (Males = 30.38 ± 1.17; Females = 30.23 ± 1.10). Participants’ racial background was Caucasian (94.0%), Asian (3.1%) and Aboriginal and Islander (2.9%). In the 21-year follow up 41.2% of respondents were single and were excluded from further analysis.

Measurement

Intimate partner violence

We measured IPV at 21 and 30 years using a modified version of the Composite Abuse Scale (CAS) [35, 36]. The CAS is a validated and widely used measure to assess the frequency of experiences of violence in intimate relationships (in either current or previous relationships) [37, 38]. It should be noted that despite similar items, these two measures have a difference in the timing of recall of each item: at 21 years respondents were asked to recall “ever happened” incidences of IPV, but at 30 years they were asked to recall their “last year” relationships. Both questionnaires ask about respondents’ current or previous relationships.

The scale consists of same 21 items (α = 0.94 for the 21 and 0.93 for the 30 year old follow up) and 4 subscales: severe combined abuse (has two items including rape and assault with a knife or weapon; possible score 0–10), emotional abuse (α = 0.91 for 21 and α = 0.90 for 30 years; has 11 items that include keeping apart from friends and family, insults, blame and verbal violence; possible score 0–55), physical abuse (α = 0.91 for 21 and α = 0.87 for 30 years; has 4 items which include kicking, slapping, hitting; possible score 0–20) and harassment (α = 0.83 at 21 & α = 0.72 at 30 years; comprises 4 items including harassing at work and over the telephone; possible score 0–20). Response options are never (=0), only once (=1), several times (=2), once a month (=3), once a week (=4) and daily (=5) [36]. After adding the scores of each subscale, recommended cut-offs were applied (severe combined abuse (≥ 1), physical abuse (≥ 1), emotional abuse (≥ 3), and harassment (≥ 2). If a respondent’s score is equal or higher than the cut-off score, they are considered as abused. Finally, respondents who experienced at least one type of IPV were recoded into abused and those who did not report any type of IPV were categorized into non-abused.

Changing partner at 21 years

At 30 years, length of relationship was measured with a question: for how long (in years) has your current live-in relationship lasted (ranged from one month to 17 years). Average length of relationship for females was 7.3 years (± 3.8) and for males was 6.3 years (± 3.6). Length of relationship was subtracted from the duration between two surveys and categorized into those who had stayed with the same partner since the 21 year follow up and those who had changed partners after the 21 year follow up.

Covariates

We adjust for a number of demographic and personal variables that may be related to both early and later IPV victimization as well as leaving a relationship. Previous research suggests that [re]victimization and staying in an abusive relationship, are related to the following factors: marital status, lower socio-economic status, presence of children, history of child abuse, and poor mental health [24, 39,40,41,42,43].

All participants were about 21 and 30 years of age at each follow up, so we did not adjust for the age differences in the cohort. Marital status was measured by the question what is your present marital status? Categories comprised single/never married, living together, married or separated. Education levels included high school or less, diploma and college and university. Having a child was dichotomized into yes and no. Participants were asked about their family income which was defined as gross income before tax. Then using the Australian National Poverty Line as a guide [44], we categorized those respondents whose income was at or below the poverty line into low income and the rest into higher income. For measuring history of sexual child abuse 21-year respondents were asked whether they had experienced being pressured or forced to have sexual contact before they were 16 years. Depression was assessed using the Center for Epidemiologic Studies Depression Scale (CES-D) which is a widely-used self-report scale [45]. It contains 20 items measuring the current level of depressive symptoms over the past week (e.g., feeling hopelessness, restless sleep, poor appetite) in a general population (α = 0.88). Response options range from 0 (rarely/less than 1 day) to 3 most of the time/5–7 days). Scores range from 0 to 60, with high score indicating a greater level of depressive symptoms. Based on the recommended cut-off score (> 16), respondents at 21 year follow-up were grouped into two categories: non-depressed and depressed [46, 47]. Sexual orientation at 30 years was measured using a single question during the last 12 months have your sexual partners been: only the opposite sex, only the same sex and both sexes. We then categorised respondents into two categories of heterosexual and homo/bisexual.

Data analysis

In Table 1, the prevalence of each type of IPV was determined across the gender groups, chi square and t-test were used to test the significance of differences. In Table 2, we performed a univariable logistic regression analysis for the association (expressed as the odds ratios with 95% confidence intervals) between each covariate and IPV at 30 years. In Fig. 1, we compared percentages of abused or not abused males and females, based on their choice to stay or leave, using chi square test and set a p-value< 0.05 for significance. In Table 3, univariable and multivariable logistic regression models were used to examine the relationship between forms of IPV at 21 years and leaving the then abusive partner, separately for males and females. Finally, an interaction term (IPV at 21 × leaving/staying) was used to examine the effect of staying/changing an abusive partner on the association between IPV victimization at 21 and 30 years. Statistical analyses were carried out using STATA-13 and SPSS-24 statistical software packages.

Table 1 Gender differences in study variables Full size table

Table 2 Univariable logistic regression analysis for study covariates and changing partner of 21-year-old and IPV at 30 year follow-up (OR; CI 95% ) Full size table

Fig. 1 Gender differences in continuity of IPV at 21 and 30 year follow ups by staying with/changing partner. Solid arrows represent statistically significant differences between the groups (p-value< 0.05). Dashed arrows indicate non-significant associations (p-value > 0.05). P-values were obtained from the chi-square test Full size image