Several methodological limitations of this study should be noted. The following section describes limitations in the sample, design, setting, procedure and measures used in the study. It also highlights potential difficulties emerging from the statistical analyses carried out in this study.
There was lack of variability in the demographic characteristic in this sample. The majority of women were middle-class Caucasians. The study also relied on self selection and it is unknown whether the participants may differ from women who chose not to take part.
Although Belsky's (1984) process model also emphasises the importance of contextual sources of stress and support as one of the main determinants of parenting in addition to parental personality and child effects, such factors were not measured and controlled for in this study. This omission limits the interpretability of findings emerging from the current study, as such factors might also account for significant variance in parenting.
In addition, the measures used in this study warrant some caution. Information on maternal personality ( ‘trait mood’) obtained from the PANAS lacked variance in that most mothers obtained scores at the high end of positive affect and the low end of
Discussion
negative affect. This sample characteristic may not reflect the general population. Furthermore, the analysis of variables restricted in range may have reduced statistical power. In addition, the PANAS is more commonly used to assess state, rather than trait mood. Given Dix’s (1991) suggestion that mood inventories should be used in parenting research, the PANAS provided an opportunity to assess mood at the level of personality. However, the suitability of such measures in parenting studies remains to be established through further studies.
The NICHD maternal sensitivity and infant behaviour rating scale is limited in its 4- point Likert scale. Given that points 1 and 4 on the scale were restricted to observed behaviours of ‘extreme’ nature, these were rarely coded. The resulting information on maternal sensitivity and infant behaviour was thus very limited in range. The lack of observed variation indicates that this scale may not have been sensitive enough in detecting the full spectrum of maternal and infant behaviour.
Although Rothbart’s (1986) Infant Behaviour Questionnaire (IBQ) is routinely used to establish second-order composites reflecting positive and negative emotionality, an examination of the internal consistency of scales revealed low values for negative emotionality. This suggests that the validity of the scales may be generally questionable. The scales constituting negative emotionality i.e. fear and distress to limitations were therefore investigated separately in the analysis.
The brief laboratory-based observation may not have been conducive to eliciting ‘natural’ parent-infant interaction. In fact, some mothers commented on how contrived their behaviour during the video recorded session of mother-infant
interaction felt. More extensive and naturalistic observations might give rise to representations of more ‘typical’ of mother-infant interaction between the dyads. However, it is of note that the assessments of mother-child interaction with this scale by NICHD studies have been reliable predictors of children’s social and cognitive behaviour (NICHD, 1999).
The analysis of interaction effects (i.e. where the effect of one independent variable on the dependent variable depends on the level of another independent variable) using continuous variables is being increasingly conducted in social science research (Aiken & West, 1991). Despite their increased use, however, statistically significant interaction effects are infrequently found because of methodological problems that interfere with the reliable detection of such effects (see McClelland and Judd (1993) for details). For instance, despite following the convention of means-centering (i.e. using standardised scores), it is likely that main and interaction effect variables remain correlated with each other which reduces statistical power. In addition, common deviations from normal distributions, which are unproblematic for linear analyses, can cause extreme reductions in efficiency in detecting interaction effects (Bates, 2001). Such reasons make it likely that some interaction effects that exist will be undetected. This indicates that the current research might be limited by Type II errors. However, findings also have to be interpreted with caution due to the possibility of Type I error. In the case of multiple regression analyses, suppressor effects might lead to the detection of interaction effects that are spurious. Suppressor effects are caused by variables that inflate the importance of other independent variables by suppressing irrelevant variance in the other independent or dependent variables (Tabachnick & Fidell, 2001). This complicates the interpretation of
Discussion
interaction effects identified in the current study. With this information in mind, the interaction between maternal negative affect and infant positive emotionality, which emerged as significant even when infant behaviour was controlled for could be interpreted as the following. The measure of infant temperament may be characterised by much statistical error; controlling for infant behaviour may be removing noise in the measure of temperament and thereby give rise to a significant interaction between maternal negative affect and infant temperament. Tabachnick and Fidell (2001) note that suppressor effects are notoriously difficult to interpret, as it is hard to identify what causes them.