Jenna Jackson and Dr. James Dee Higley, Psychology
Environment can shape temperament, although temperament is thought mainly to be biologically inherited. Thus, mothers’ temperament is often similar to the infants’ temperament. Previous studies in rhesus monkeys have shown that infants have a similar temperament to their mothers, and that there is a potential genetic influence.1 In fact, the rh5-HTTLPR (serotonin transporter gene) and the rh-MAOA-LPR gene have been linked to affecting temperament, interacting between the mothers and infants. However, only male infants exhibit this effect; female infants’ temperament was not correlated with their mothers’ temperament.
However, there is still a question of whether or not the mother shaped the environment, thus influencing the temperament, or if it a mainly genetic contribution to the infants’ temperament. We propose to investigate the effect of environment versus biology on temperament in infant rhesus macaques (Macaca mulatta) by comparing mothers’ temperament to their infants, and then comparing cross-fostered (adopted) mothers and biological mothers. Adopted mothers do not share the same biological component, but would shape the environment of the infant. Biological mothers would do both. This will assess the impact of environment versus the biological factors on temperament in infant rhesus macaques.
The non-human primate model allows us to better assess the degree of biological and environment influence on temperament because it offers more control. This is necessary when studying the delicate balance of genetics versus biology, as it gets quite complicated. For this project, we used data from the California National Primate Center, at UC-Davis. The testing procedure, known as the BBA (bio-behavioral assessment) is an on-going research project. All of the information has been complied into a database, which was used for this project. We went down to the California National Primate Center to work on this study during the summer months.
Infants between four and six months of age were selected at the UC-Davis National Primate Center for testing. Some of the subjects were cross-fostered, or ‘adopted.’ During the first few days of the infants’ life, the cross-fostered infant is removed from the mother and given to a new mother who also just had an infant. The cross-fostered mother and infant are observed to ensure that proper bonding has occurred. Cross-fostering is fairly rare, so the sample size for the cross-fostered subjects was limited.
Infants underwent a 26 hour period of separation from their mother, and were tested in a series of tests known as a bio-behavioral assessment (BBA). The same trained professional observes and records all behavior to standardize the tests and reduceobserver bias. Temperament was analyzed in four categories: vigilance, gentleness, confidence, and nervousness. After the other bio-behavioral assessments are completed, the infant is reunited with the mother. Females that underwent testing eventually gave birth to other infants, and those infants were also assessed. This study compared mothers that have been through the BBA testing process and their infants.
Basic correlation between the mother and infant were assessed, as well as a regression analysis for the four temperament categories. A global F-test then compared the biological and cross-fostered mothers’ and infants’ temperament regression analysis.
Preliminary analyzes show no significant difference between the cross-fostered mothers and their infants and biological mothers in any of the temperament areas. Global F-tests comparing regression analyzes of temperament of mothers and infants was statistically insignificant. Furthermore, when controlling for the sex of the infant, there was still no significant difference between biological mothers and cross-fostered mothers in affecting the mothers’ temperament on the infants’ temperament. Thus, preliminary analysis show there is no substantial difference in cross-fostered and biological mothers, suggesting that temperament is not affected more by genetics, but environment.
This finding does not fully explain the relationship between genetics and environment in temperament when comparing mothers to their infants. Since the results were not significant, it suggests that the environment shapes the temperament more than the biological inheritance from mother to infant. However, this preliminary finding contradicts previous findings. Thus, it appears that the interaction is more complicated, and there are possible lurking variables that need to be addressed. However, due to a small sample size, that limits the potential analyzes until more cross-fostered subjects are tested.
Further analysis may explore the effect of parity and the serotonin transporter gene, and other important factors. Furthermore, the small sample size in cross-fostered subjects may decrease the power or bias the results. A larger sample size may highlight some of the key aspects of the mother-infant temperament interaction. This needs to be examined before any conclusions on the significance of environment in temperament may be addressed. It is important to note that this project will continue to be researched, and more advanced analyzes will be run to adjust for sample size. Another examination of how the serotonin transporter genotype interacts with the mothers’ and infants’ temperament comparing cross-fostered and biological mothers will help explain the biological and environmental effects of temperament.
This research project is part of many being conducted at the UC-Davis National Primate Center. Initially, a different project was proposed, but due to limited time and resources at the Primate Center this study was conducted. However, we were able to participate in other research projects as well because of this ORCA grant. We would like to express appreciation to all those who helped to make this possible.
References
- Sullivan, E.C., Mendoza, S.P., & Capitanio, J.P. Similarity in temperament between mother and offspring rhesus monkeys: sex differences and the role of monoamine oxidase-a and serotonin transporter promoter polymorphism genotypes. Developmental Psychobiology. 53(6). 549-563.