Birth Rate and Generation Time
For most purposes,
United States,
it
,
a generation is taken to be 30 years.
is
In the
currently a bit shorter, but 30 years is
still taken as a convenient simplification.
For the Marshall
Islands, the Five Year Comprehensive Health Plan gives a breakdown
by age of mother (Table V-3, page 120) that shows the age of
mothers at the “average” or "middle" birth to be only 23-24 years.
However, no information is available on the age of fathers, who
seem likely to be older than the mothers, and in any case the
usual 30 year interval is used here.
It should be noted that to
the extent the Enewetak generation is actually shorter, this tends
to overestimate dose to the parents of the average child, and
thus the genetic health risk estimates.
While genetic risk estimates may be expressed per live birth,
thus avoiding any assumptions about future birth rates, it is
helpful’ to attempt to estimate the total risk for the entire next
generation of the Enewetak people.
As a minimum, we might simply
assume a ‘“‘replacement" birthrate of 453 live births over the next
30 years, or about 15 births per year.
AS amaximunm, we might
assume that the average birth rate in the entire Marshall Islands
for the 20 years from 1955 to 1975 might apply to the Enewetak
population for the next 30 years.
From Table III-1 of the final
draft of the Marshall Islands 5-year Health plan, one can calculate
that the average yearly birth rate for the period (it seems
remarkably stable over this period) is 39.7+ 4 births per 1,000
of population; for practical purposes 40 per 1000 or about 11 per
year for the 273 people assumed to return to Enewetak and about 7
per year for the 180 people assumed to return to Enjebi.
Of course, should the present birth rate and current population
growth rate of the order of 3-4% per year continue, the absolute
numbers of births will grow during the coming 30 years.
Assuming
a 4% growth rate, the Enewetak population may include 816 people
15 years from now, and about 33 births might be expected that
year, while in thirty years there would be almost 1,500 persons,
with well over 60 births per year.
It seems unlikely that the
population will grow to this extent; in view of the uncertainties
involved, perhaps a reasonable assumption would be that there
will be not more than roughly 1000 births in the population
during the next 30 years.
With exponential population growth, roughtly one-half of the
births expected over 30 years will occur during the first 20 years;
the remainder will occur during the final 10 years.
In view of
the uncertainties involved, it seems reasonable to assume as an
upper credible limit that there will be 1000 births, the average
accumulated parental dose for which will be that accumulated for
the first twenty years.
However, the doses were calculated for
50 years, and since these are not enormously larger than the 20
year integral doses
(see Fig.3 of Dose Assessment),
they are used
here as upper bound estimates of the doses of genetic significance
in calculating genetic risk.