(A) Those governing the formation and dissolution of unions in the reproductive period.
1. Age of entry into sexual unions.
2. Permanent celibacy: proportion of women never having sexual union.
3. Part of the reproductive period spent after or between unions:
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(a) When unions are broken by divorce, separation or desertion;
(b) When unions are broken by death of husband.
(B) Factors governing exposure to intercourse within unions!
4. Voluntary abstinence.
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5. Involuntary abstinence (from impotence, illness,| unavoidable but temporary separations).
6. Coital frequency (excluding periods of abstinence).
II. Factors Affecting Exposure to Conception (“Conception| Variables”):
7. Fecundity or infecundity, as affected by involuntary causes.
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8. Use or non-use of contraception.
(a) By mechanical or chemical means;
(b) By other means.
9. Fecundity or infecundity as affected by voluntary causes (sterilisation, sub-incision, medical treatment, etc.).
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III. Factors Affecting Gestation and Successful Parturition (“Gestation Variables”):
10. Foetal mortality from involuntary causes.
11. Foetal mortality from voluntary causes.
It should be noted that any social or cultural factor which affects fertility must do so through and only through one or more I of the “intermediate variables” listed above.
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Each of these eleven intermediate variables can have either a positive or a negative effect on fertility. For instance, if in a society a significant proportion of couples successfully practice contraception, it has a minus value with respect to the variable; and if the practice of contraception is absent in a society, this variable has a plus value.
The fertility level in any society is determined by the combined effect of all these eleven variables. “All of these variables are present in every society each one can operate to reduce or to enhance fertility. If abortion is not practiced, the fertility value of variable 11 is plus.”
In the sections, that follows, only the more important of the “intermediate variables” are taken up for detailed discussion, for these are considered to be largely responsible for determining the levels of, and differentials in, fertility.