Estimates of fertility in Protestant Barra and Catholic Harris, islands in the Outer Hebrides, over the period 1856–1985, show that in both islands fertility declined, although marital fertility was generally greater than in Scotland as a whole, and illegitimate fertility was less. However, in Barra during 1966–75 there were pronounced rises in all the indices; illegitimate fertility showed the smallest rise. The publication of the papal encyclical Humanae Vitae may have played a part in this change in fertility, although other, probably local, factors must have been acting, as the first rise in numbers of births occurred before the publication of the encyclical. Other than this transient rise, the religious difference between Harris and Barra had little effect on changes in fertility over the whole period.
Indices of proportions married showed declines until after World War 2, followed by steep increases. The marked decline in fertility in both islands may be due in part to this low nuptiality. In addition, a trend of occupation away from traditional crofting and fishing towards more skilled mercantile and professional employment may have played a role.