Age-related cognitive decline is an increasingly pressing concern in public health, which may begin in early adulthood and accelerate with increasing age1,2. While much research is still investigating the precise mechanisms of cognitive ageing, previous studies have identified several contributing factors, including for example genetics, general health and lifestyle choices3,4,5,6,7. Cognitive decline is a major public health concern on both individual and societal levels due to its association with diminished well-being and health-related quality of life8,9 as well as increased caregiving burden10 and health- and long-term care costs11. Therefore, it is important to investigate potential avenues to support cognitive health across the lifespan.
With regards to individual lifestyle factors, previous research has shown that, for example, an adherence to a Mediterranean- or DASH-style diet rich in vegetables, fruits, whole grains and unsaturated fats; sleeping 7–8 h per night with limited night-time fragmentation; regular participation in social clubs or volunteering; enrolment in adult-education courses; or employment in cognitively demanding occupations, are related to reduced or delayed deterioration of cognitive functioning12,13,14. Another potentially protective factor of cognitive health, which has received so far relatively little attention, is pet ownership. Approximately 38% of Europeans own pets, with similar rates of pet ownership estimated in older adults aged 50 and above14,15,16,17.
Numerous studies have linked pet ownership to a range of physical and mental health benefits18,19,20. When it comes to pet ownership in later age specifically, it has been shown to have a positive effect on older adults’ well-being and feelings of companionship19 as well as on their physical health. A systematic review by Gee and Mueller20 found robust evidence linking pet ownership to better physical health, in particular, cardiac health and physiological responses to stress, such as lower blood pressure or lower risk of fatal cardiac events in patients with hypertension. Furthermore, the review revealed that pet ownership had important psychosocial benefits. Specifically, older pet owners perceived lower rates of loneliness, and dog ownership in particular was linked to increased physical activity, which in turn related to positive social outcomes.
A further systematic review by Hughes and colleagues21 similarly concluded that human-animal interactions can benefit older adults’ physical but also mental health, including improvements in their quality of life and levels of depression and anxiety. Additionally, they identified five studies suggesting that the human-animal interaction could improve cognitive health and eight studies which found no effect on cognitive functioning. Most of the included studies focused on animal-assisted interventions and the one identified study which focused on pet ownership specifically, albeit cross-sectional, found significantly better executive functioning in pet owners22, suggesting that long-term pet ownership might be associated with better cognitive health. The empirical evidence in this newly emerging field of research is still limited, often mixed, and primarily derived from studies involving animal-assisted interventions. However, as outlined in the following, there are several reasons to hypothesize that pet ownership may be associated with slower cognitive decline.
Firstly, an increase in physical activity and a decrease in loneliness are both factors known to be related to reduced risk and a slower rate of cognitive decline23,24 Secondly, pet ownership has also been associated with increased social interaction25 and reduced anxiety symptoms26. These factors, as highlighted in meta-analyses27,28 are linked to a lower risk of cognitive impairment in older adults. Thirdly, the presence of a companion animal has further been associated with reductions in stress levels through the decrease in cortisol levels and heart rate during stressful situations29. Finally, pet ownership appears to not only lower the blood pressure response to mental stress30 but has also been associated with lower systemic blood pressure31. In sum, these findings highlight the importance of maintaining low stress levels and weak physiological responses to stress (through the excessive cortisol production and the associated hippocampal damage), as they have been linked to reduced risk of cognitive impairment32,33,34,35 and to less steep cognitive decline35,36,37. Although pet ownership is linked to many factors that have been also related to reduced cognitive decline, there is still only limited longitudinal and population-level research regarding the direct link between pet ownership and cognitive outcomes.
There is cross-sectional evidence pointing towards pet ownership being associated with higher levels of cognitive performance in several areas including processing speed, attentional control or episodic memory38,39. In terms of longitudinal studies, the empirical evidence appears to be inconsistent and dependent on various factors. For example, Branson and Cron40 found no relationship between pet caretaking and the risk of developing a mild cognitive impairment over a 12-year timespan after adjusting for potential confounding factors in their study of 2251 older adults aged 50 and above. This could be due to the fact pet caretaking might entail limited exposure to pets compared to their ownership, and hence some of the pet ownerships’ benefits associated with effects on cognitive decline were not experienced by all participants.
Applebaum and colleagues41, on the other hand, found that participants aged 65 and older who owned pets for more than 5 years had higher mean composite cognitive scores compared to those who owned pets for a shorter period of time or not at all. Yet, they found no association between cognitive decline and pet ownership in adults aged between 50 and 65, suggesting that there might be age group differences in the association of pet ownership with cognitive functioning. Therefore, further and fine-grained investigations are necessary to further elucidate the role of age. Lastly, Li and colleagues42 found slower cognitive decline over 8 years in pet owners compared to non-owners in measures of verbal fluency and verbal memory. Their additional analysis showed that pet ownership was associated with a slower decline in these domains only in individuals living alone compared to those living with others.
To better understand the pattern of associations between pet ownership and cognitive decline, it would also be of interest to separately examine the contributions of owning different species of pets. For example, Friedmann and colleagues43 found, with modest to moderate effect sizes, that pet owners, and especially dog owners, experienced slower cognitive decline over ten years across various facets of cognitive functioning, such as memory, executive functioning, language, psychomotor speed, and processing speed, compared to non-owners. Cat owners showed less decline in memory and language functioning. Additionally, dog owners who regularly walked their dogs had slower cognitive decline than those who did not, suggesting the important role of physical activity through pet ownership for cognitive health. However, the fact that several domains of cognitive functioning showed slower deterioration also in cat owners and dog owners who did not walk their dogs, implies that other factors besides physical activity per se may be at play. Despite the potential recall bias, as the participants had to recall having a pet in the past, these findings highlight the need to further scrutinise in detail the associations of different pet species with cognitive decline.
To build on these previous findings and to overcome some of the remaining research gaps outlined above, the present study will first aim to investigate the association of overall pet ownership with cognitive decline over a period of 18 years in a large European population-based sample in order to achieve greater statistical power, higher precision, and greater generalisability, compared to previous studies. Additionally, we will also examine the potential moderating role of younger and older age groups as well as the specific associations with different pet types. Based on the empirical literature outlined above, we hypothesize that: (1) Pet ownership will be associated with slower cognitive decline, (2) Older age groups will have a stronger association between pet ownership and reduced cognitive decline than younger ones, and (3) There will be species-specific differences in the strength of associations between the pet ownership and cognitive decline with larger associations for dog and cat owners compared to owners of pet resulting in less deep human-animal interactions.