Sexual inhibition, from prudential motives is the real cause in New Zealand.
Sexual inhibition implies well-developed self-control, the very force in which almost all defectives are most deficient, and the absence of which makes them criminals, drunkards and paupers.

In almost all defectives too, prudence is conspicuous by its absence.
The only moral force we know of, that has curtailed, or will curtail, the family within the limits of comfortable subsistence, is sexual inhibition with prudence.

But this force is absolutely impossible amongst defectives.
It is not only a powerful force among the normal, but with us to-day it is powerfully operative.

Amongst the defectives it does not and cannot exist.
Apart from observation and statistics, therefore, it can be shown that the birth-rate amongst the unfit is undisturbed.

They marry and are given in marriage, free from all restraint save that of environment, and worst of all they propagate their kind.
Dr.Clouston says (Clinical Lectures on Mental Diseases, 4th Ed., p.
330) "As we watch children grow up we see that some have the sense of right and wrong, the conscience, developed much sooner and much stronger than others; just as some have their eye teeth much sooner than others; and looking at adults, we see that some never have much of this sense developed at all.