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About 80% of human females cradle their infants on the left side of the body. XIANNING & CHAMBERLAIN (1990) found a similar left-side bias for chimpanzees and two other ape species. In the present paper, however, only a slight bias was found which was in contrast - right-sided. The combined results of the two studies do not differ from an homogeneous distribution between 0 and 100% right-sided. When data for 3 ape species and 3 studies are pooled, a significant bias is also absent. Even if a left-side bias were present in apes, comparability with humans is limited. The possible functions of a left-side bias in humans (better hearing of the mother's soothing heartbeat and better visual contact with the left side of the mother's face) might not apply to chimpanzees. For there are relevant behaviour differences between chimpanzees and humans. These concern different ways of holding and carrying, rare crying by ape infants, and rare facial orientation in apes. Moreover, most of the ape infants involved were older than the age at which the human bias is found. For the time being, no convincing evidence for a bias in apes exists.

Abstract The present study examines the effects of gestational age and birthweight status on the development of infant and maternal behaviour as well as the (mutual) predictability between partners during face-to-face interaction at 6, 12 and 18 weeks of corrected age. Subjects are healthy fullterm infants (N = 6) and three groups of healthy preterm infants: small-for-gestational age (N = 6), and appropriate for gestational age, the latter being born after a pregnancy duration of less than 32 weeks (N = 6) or between 32 and 34 weeks (N = 6). Using dyadic sequential analyses, based on log-linear modelling and information statistics, for each individual infant-mother pair at each age, the effect of both partners on the behaviour of the other was quantified, while accounting for autocorrelational effects. In the majority of cases the interactions could be labelled as showing 'bidirectionality', particularly at 18 weeks. Mothers were more likely to be influenced by the previous behaviour of the infants than vice versa. No group differences were apparent in the way mothers were influenced by their infants. However, the small-for-gestational age preterm infants were less likely to be influenced by maternal behaviour, particularly at 6 and 12 weeks of age. A lower variability was the most common finding in infants who were not predictable from the previous behaviour of the mother. At 6 and 12 weeks they were less expressive and more often showed a monotonous behavioural pattern in which 'looking at mother's face without positive expressions' was shown during most of the interaction. As this lack of variability was more often found in SGA preterm infants, they were less predictable from their mother's behaviour than the fullterm and AGA preterm infants. These data suggest that having a low birthweight for gestational age may be a risk factor for a lack of 'bidirectionality' during early mother-infant interaction.

In recent studies on early communication a preference seems to exist for applying statistical methods to behaviours that have in some way been scaled. As an alternative, two methods for analyzing the interaction between two sequences of nominal data are discussed: an information-statistical analysis as developed by VAN DEN BERCKEN & COOLS (1980a) and log-linear modelling. Both methods make use of contingency (transition) tables, which should be constructed according to common requirements that are often ignored in practice. Firstly, the relationships between the number of behavioural categories, the length of the recording, the complexity of the model and the reliability of the statistics need to be considered. Secondly, choices have to be made regarding the use of either time or event sequences. Although these requirements are explained and illustrated using theoretical considerations and data on early mother-infant communication in humans, the application of these models may be much broader. Since informational and log-linear statistics are mathematically related, both can be used to describe the same measures of directionality in the interaction of individual pairs.