Parental emotional ties toward a child, particularly an infant, are termed bonding. Perinatal psychiatry cast light on parental bonding in the 1990s. There are mothers who experience dislike, resentment or hatred, and/or desire for permanent abandonment toward their infant and hope for the child to disappear [1, 2]. Bonding disorders toward infants may have serious and long-term consequences for the child’s development and mother-child relationship . An international position paper on mother-infant (perinatal) mental health  notes that “a small minority of mothers may suffer from emotional rejection of the infant, which, together with psychosis and suicidal depression, is in the first rank of severity in this area of psychiatry.”
There are self-report instruments to measure parental (particularly maternal) bonding disorders. One of them is the mother-to-infant bonding scale (MIBS) . A Japanese modification of the MIBS (MIBS-J), developed by Yoshida et al. , has been widely used in clinical and research settings in Japan (e.g., Ohashi et al.  and Baba et al. ). However, caution is required when the MIBS-J is used in research settings. First, the MIBS-J is often repeatedly used routinely across short periods of time for individual women (e.g., during postnatal hospitalization, 1-month check-ups, and 4-month infant check-ups). However, the repeated use of a psychological measure is usually accompanied by reduced validity [9, 10, 11], and the appropriateness of using all the items of the MIBS-J at multiple time points is not clear. Second, bonding disorders are not only a problem for mothers, and bonding research should also focus on fathers [12–18]. Currently, only mothers have been included in many studies. Thus, the target should be expanded to include fathers. However, the appropriateness of the use of the MIBS-J among perinatal fathers remains to be investigated. Third, some of the 10 MIBS-J items are quite highly skewed [6, 14]. Consequently, most of the general population of parents would report “no problem” to these items. Indeed, there may be little clinical significance in using all 10 items for all cases, as is currently the case.
These methodological issues require validation and sophistication of the MIBS-J factor structure. The MIBS-J factor structure has been reported in at least two studies [6, 14]. Yoshida et al. conducted a longitudinal study of postpartum mothers (n = 554) and collected responses at three different time points: 5 days, 1 month, and 4 months after childbirth . They found that seven, eight, and seven MIBS-J items showed skewness > 2.0 at 5 days, 1 month, and 4 months after childbirth, respectively, although there was no skewness of the MIBS-J items after log-transformation. They further conducted combined exploratory and confirmatory factor analyses (EFAs and CFAs, respectively), and a two-factor structure (lack of affection [LA] and anger and rejection [AR]) was extracted. However, this procedure might have blurred the differences in terms of the factor structure of the scale across the time periods because the data obtained from the three time points were combined to create a single data set. Subsequently, separate CFAs were conducted for the data from the three time points. Acceptable fitness of the model was found with the data at 5 days after childbirth (comparative fit index [CFI] = .922). However, fitness was far less than required (CFI > .95)  at 1 (CFI = .889) and 4 (CFI = .905) months after childbirth. Scrutinizing items one by one separately revealed that standardized factor loading was 0.09 for item 2 (“scared or panicky”) at five days after childbirth and 0.29 for item 7 (“wish baby was different”) at one month after childbirth. These findings suggest that the factor structure of this model might differ across the three time points. The means of the two subscale scores decreased with time; however, Yoshida et al. did not measure the factor mean invariance across the time points . A few years later, Kitamura et al. collected data using the MIBS-J from a cross-sectional sample of fathers (n = 396) and mothers (n = 733) with children aged 0 to 10 years . They identified a two-factor structure of the MIBS-J items, which was quite similar to that reported by Yoshida et al. . They found a good fit of the model with the data in terms of configural invariance (CFI = .956 and root mean square error of approximation [RMSEA] = 0.033). However, they did not report on measurement invariance.
The use of a psychological measure requires confirmation of configural, measurement, and structural invariance of the factor structure of the measure. Thus, the factor structure of the measure should be stable between participants with different demographic features (e.g., fathers and mothers), as well as across various observation time points. If these assumptions do not hold, items of the measure do not have the same meaning and may cause bias in the assessment, and a comparison of the measured scores does not make any sense. The stability of the factor structure is confirmed through several steps , and the subsequent steps are endorsed only when the preceding steps are accepted. If one step is rejected, the next step should not be performed. Basic stability is termed configural invariance. Each group (e.g., fathers vs. mothers) should have the same pattern of items and factors (first step). Moreover, factor loadings for similar items (metric invariance, also known as weak factorial invariance; second step), intercepts of similar items (scalar invariance, also known as strong factorial invariance; third step), residuals (errors) of similar items (residual invariance, also known as strict factorial invariance; fourth step), variances of similar factors (factor variance invariance; fifth step), and the means of factors (factor mean invariance; sixth step) should be invariant across groups. The second to fourth steps are termed measurement invariance, and the fifth and sixth steps are termed structural invariance.
Such confirmation mentioned above is necessary for the MIBS-J to be used in clinical and research settings. This study aimed to select and to validate MIBS-J items suitable for mothers and fathers at the three time points (postpartum hospitalization period and 1 and 4 months postpartum) commonly used in clinical practice in Japan to enable a continuous assessment of bonding disorders among perinatal mothers and fathers over time and to minimize the burden on respondents.