Rhesus monkeys develop attachment behaviours and complex social interactions from birth and remain highly dependent on their mothers for some time in a manner similar to those of human infants. They demonstrate a social hierarchy that is similar in many ways to those of humans.

Social rank is transmitted maternally and mothers, siblings and aunts care for, and reinforce, a young monkey's place in the hierarchy.

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Higher status monkeys have better access to resources (social partners, high value food items and mating partners). Lower status monkeys have more adverse stress reactivity, and less disease resistance.

Rhesus monkeys are able to complete complex cognitive tasks and cognitive tests designed for human children can be completed by them demonstrating reflex, sensori-motor and motor skill development, learning, cognitive flexibility and levels of impulsivity.

Rhesus monkeys are neurologically, anatomically and physiologically similar to humans. Rhesus monkeys can be raised in controlled and experimentally-manipulated environments.

For decades the American National Institute of Health has been studying rhesus monkeys looking at the multi-generational effects of adversity and non-adversity.

In these experiments, monkeys raised in adverse conditions are removed from their mothers at birth and reared in a nursey with human caregivers for 40 days.

They do not have contact with their parents at all. After that they are allocated to groups that have different amounts of peer contact until they are weaned at eight months.

Note that these moneys were not severely deprived - they had constant contact with humans in their first month of life and daily contact with peers throughout their early years. Monkeys reared normally stay with their mothers until eight months of age. At this point all are moved into larger social groups (with their mothers and other family members) where they mix with others from all the different experimental groups.

This allows genetic siblings to be compared across the different rearing experiences, providing evidence on the impact of maternal separation, as well as the impact of support focusing on the children and/or on the parents.

Analysis demonstrated that mothers who were reared in the adverse environment had higher rates of early infant mortality and their children were more likely to be unhealthy.

Offspring who were mother-reared by a mother who herself was mother-reared were more likely to have a higher social status in the rhesus dominance hierarchy.

The impact appears complementary; in other words, offspring who are mother reared show a greater advantage in health and social status if their mother was also mother-reared compared to those who were mother-reared by a mother who was nursery-reared.

The researchers suggested there might be two mechanisms at play here. First, mothers who were mother-reared might have biological changes that improve the in utero environment for their offspring who are therefore born more advantaged.

The other explanation involves parenting behaviours. Mothers who were themselves mother-reared might have more beneficial parenting behaviours that served to advantage their offspring.

Further statistical analysis of the data suggested that the parenting explanation is the most significant of these explanations.

Thus, it appears that parenting behaviours are most important in transmitting advantage to the offspring generation. These inter-generational effects of maternal advantage were observed to persist until offspring adulthood.

Of course, experiments with humans cannot be performed in the same manner, but there is human evidence that aligns with these findings.

Firstly is the Perry Preschool Project (supported by a range of other early intervention studies) that demonstrates the advantages resulting from early enrichment programmes.

Children in the Perry Preschool Project demonstrated significant improvements in education, health and social behaviour that lasted through to adulthood, and are now being observed into the next generation. Children of the Perry Preschool graduates are demonstrating better health, education and employment outcomes.

In a similar manner, studies of the Romanian orphans following the fall of the Ceaucescu regime show that severe deprivation in the early years cannot be overcome unless intervention occurs very early in life.

The researchers suggest that the key message regarding the inter-generational transmission of advantage (and its converse, disadvantage) needs to be applied to humans so that we continue interventions over sustained periods of time in order to make lasting changes in the lives of vulnerable families and communities.