Since 2001 experiments have been conducted in the field of stem cell research in which human neuronal stem cells are implanted in the brains of non-human primates. With the aid of such experiments the goal is to investigate whether stem cells can replace dead or functionally impaired brain cells, thereby making it possible to restore the functioning of damaged areas of the brain. In the long term this research hopes to develop new therapies for neurodegenerative diseases such as Alzheimer's or Parkinson's with the aid of stem cell technology.

In the opinion of many scientists this line of research also gives rise to a special ethical problem: if the primates used were to acquire human-like cognitive and emotional capacities as a consequence of the injection or transplantation of human nerve cells, their (further) use in harmful or painful experiments would not be ethically acceptable.

While many researchers assess the probability of such a "surge in mental capacity" as minimal, it cannot be entirely ruled out. Given that knowledge about the "normal" emotional and psychological life of primates is currently still slight, there is also a lack of criteria that could be used to identify such a change with any degree of certainty.

In view of the looming ethical problem a multidisciplinary research panel set up in 2004 drew up a catalogue of measures aimed at minimising the risk of "humanising" test animals. These require that special consideration be given to the following points before conducting such a procedure:

(I) the proportion of grafted human cells (this should not be too large relative to the brain volume),

(II) the stage of development of the test animal (the procedure should not be performed in too early a stage of development),

(III) the species of ape (the risk of humanisation could be greater with anthropoid apes than with other species of monkey),

(IV) the brain size (this in turn influences the proportion of human cells),

(V) the place of insertion (the foreign cells should not be injected or transplanted into those areas that are responsible for cognitive and emotive capacities) and

(VI) the cerebral pathology (humanisation appears more probable if the inserted human cells are intended to replace the function of heavily damaged areas of the brain).

The panel recommends that research projects involving the injection or transplantation of human cells into the brains of non-human primates should be subject to a special assessment procedure.