Abstract
Ageing is associated with cognitive decline. While many areas of cognitive functioning are affected, the
primacy of two domains, Processing Speed (PS) and Executive Functions (EF), has been frequently
debated. There is limited literature on the relative contributions of genetic and environmental factors on
age-related cognitive decline. This thesis uses the twin design to address some of the important issues in this field.
Using data from the Older Australian Twins Study (OATS) and quantitative genetic modelling, the extent of genetic influence (heritability) and unique environmental influences on multiple measures of PS and EF, and General Cognitive Ability (GCA) were investigated. The covariation between five PS measures and GCA was found to be explained by one common genetic factor, suggesting that the relationship between PS and GCA is entirely due to shared genetic influences. The phenotypic association between four measures of EF was modest, suggesting that it is a multi-dimensional construct, but the genetic association between the measures was also explained by a common genetic factor, shared with that for GCA. These findings help clarif,z the genetic relationships between measures of PS, EF, and GCA, and can inform future studies in identiffing specific genes involved in specific cognitive abilities.
Investigation into the mediating effects of PS and EF on age-related variation in memory, found both to
be significant mediators, with PS conferring much stronger effects. This finding was cross-validated
across three samples, confirming the important role of PS as a marker for cognitive ageing. The
relationship of PS and some environmental factors also supporled its consideration as a biomarker for
cognitive ageing.
The associations between three leisure activities (cognitive, physical, and social) and various
health/medical factors, and performance in specific cognitive domains and global cognition were also
examined, using the discordant monozygotic twin design. A number of cardiovascular, frailty and sensory factors were identified that explained the discordance in cognitive function.
This research has therefore identified important genetic influence and environmental factors to promote
our understanding of neurobiological mechanisms in cognitive ageing, and has practical implications for
interventions to slow age-related cognitive decline.