A study conducted by Higley et al., 1991 involving twenty-two rhesus monkeys (all aged 50 months) was set up to investigate if their early life experience affected their alcohol consumption.
The twenty-two monkeys were split into two groups of eleven:
GROUP ONE (also known as the PEER-REARED GROUP):
The eleven monkeys that made up this group had been reared for the first six months of their lives with other monkeys of their own age but without access to adult monkeys (i.e. they had been caused early-life stress due to maternal separation); this had the effect of reducing their exploratory behaviour and increasing their fear-related behaviours
GROUP TWO (also known as the MOTHER-REARED GROUP):
The eleven monkeys in this group were reared for the first six months of their lives by their mothers.
N.B. AFTER THE FIRST SIX MONTHS OF THEIR LIVES, BOTH GROUPS OF MONKEYS WERE TREATED IDENTICALLY.
THE FIRST PHASE OF THE EXPERIMENT:
For ONE HOUR PER DAY, and for four days per week, all twenty-two monkeys were given free access to alcohol (a 7 per cent ethanol solution).
RESULTS FROM PHASE ONE OF THE EXPERIMENT:
When the two groups of monkeys were compared, it was found that the PEER-REARED monkeys (i.e the maternally deprived monkeys from GROUP ONE), consistently CONSUMED SIGNIFICANTLY MORE ALCOHOL (to levels producing intoxication) than the monkeys from GROUP TWO (i.e. the monkeys who had been raised in a normal way – by their mothers).
THE SECOND PHASE OF THE EXPERIMENT:
In phase two of the experiment, mother-reared monkeys (GROUP TWO) were caused to experience elevated stress levels by being subjected to social-isolation.
These (GROUP TWO) monkeys, as a result of this increased stress, increased their level of alcohol consumption to a level almost equal to that consumed by the peer-raised (GROUP ONE) monkeys (to levels producing intoxication) during the first part of the experiment.
OTHER FINDINGS FROM THE EXPERIMENT:
- Those monkeys who consumed most alcohol also displayed a higher incidence of distress-related behaviours.
- Peer-reared (GROUP ONE) monkeys were found to produce more biological indicators of stress(e.g. increased levels of the stress-related hormone cortisol) than the monkeys from GROUP TWO (mother-reared monkeys).
- In normal living conditions, monkeys reared without a mother or other adults in early life are more likely to suffer stress and to develop fear-related behaviours than monkeys reared by their mothers and this, in turn, increases their (i.e. the maternally deprived monkeys) alcohol consumption to levels significantly higher than the levels consumed by mother-reared monkeys.
- Stress caused to monkeys by socially isolating them increases their level of alcohol consumption significantly, even when they have had the early life relative security of having been raised by their mothers and are not, therefore, especially sensitive to the adverse effects of stress.
By extrapolation, the implications pertaining to possible contributing factors to human dependence upon alcohol are clear.
Higley, J.D., Suomi, S.J. & Linnoila, M. CSF monoamine metabolite concentrations vary according to age, rearing, and sex, and are influenced by the stressor of social separation in rhesus monkeys. Psychopharmacology 103, 551–556 (1991). https://doi.org/10.1007/BF02244258
David Hosier BSc Hons; MSc; PGDE(FAHE).