This research programme investigated how adversity experienced early in life affects cognition in adulthood in two different long-lived species, humans and European starlings. Previous research suggested that there might be cross-species similarities in the way early-life adversity shapes cognition, but the extent of commonalities had not been systematically investigated. We focused on three cognitive domains where we had some evidence that early-life adversity may be important: impulsivity, dietary cognition, and threat-related cognition. For each domain, we sought to characterise how the outcome relates to different facets of early-life adversity. Early-life adversity was measured using socioeconomic and familial variables in humans, but in young starlings it was experimentally manipulated via cross-fostering and hand-rearing siblings apart so that they experience different early histories. To measure the adult outcomes in each cognitive domain, developed behavioural paradigms with directly analogous versions in the two species. We also examined whether telomere length, a cellular measure of cumulative stress exposure, statistically mediates the relationships between early-life adversity and the cognitive outcomes, thus testing recent theoretical models based on psychological adaptation to ones own physical state. As well as describing associations between early-life adversity and adult outcomes, we focused on adaptive questions: do the observed effects of early-life adversity simply represent pathology, or can they be considered as adaptive responses? Thus, we tried to move beyond cataloguing the cognitive consequences of early-life adversity, and begin to explain them.
We were able to show, in the starlings, that different components of early-life adversity have different effects on different adult traits. That is, each trait is affected by different signatures in early experience; in at least some cases, rather than just saying individuals have had more or less adversity, you need to investigate exactly what experiences they have had. We also found some evidence that starlings (and maybe people) are sensitive to their own physiological conditions (as reflected in their phenotypic age or developmental telomere attrition), in setting their behaviour and cognition. We found clear evidence that early experience affects telomere dynamics, but much less evidence that adult experience continues to do so. In fact, telomere length through adulthood mainly just tracks on along the path mapped out in early life. Finally, we found abundant evidence for experience, including but not limited to early life experience, affecting fat storage and weight gain, not by making individuals consume more calories, but by making their metabolic expenditures different. This has important implications for human obesity and food insecurity.
