The first basic finding of the three studies listed below was to provide evidence that British Mormons display broadly the same unique pattern of fertility as US Mormons.
The pattern can be found summarized in Rodney Stark's The rise of Mormonism (2005) - and especially draws on the research of Professor Tim Heaton of Brigham Young University, Utah, USA.
The similarity implies that the cause of the unique Mormon fertility pattern in two different countries is the Mormon religion specifically, rather than some factor specifically to do with the nature of the US Mormon population, such as a founder effect, or the non-religious social situation in Utah and the adjacent states of the Mormon heartland.
The second basic finding was consistent with religious factors being a significant cause of Mormon fertility, which was the self-reported reasons for fertility decisions: the most strongly supported reasons were consistent with Mormon church teachings, some of which are either distinctive to Mormonism or else more strongly emphasised among Mormons than other denominations.
However, at present it is not possible to be very precise about exactly which of these theological factors are the most important in fertility decisions.
The unique Mormon pattern of fertility.
1. Larger than replacement level numbers of children.
(That is to say significantly higher than 2.1 children per woman, on average).
2. Despite widespread use of contraception. This implies that for Mormons family size is usually chosen deliberately.
(By contrast, other religious groups with significantly above-replacement fertility do now allow contraception.)
3. In a group of people who are more educated than average and economically more successful than average.
4. Average fertility (numbers of children) tends to be highest among the most educated and wealthiest Mormons.
(This is a reversal of the normal pattern seen in developed nations, where the most educated and wealthiest people have the smallest average family size.)
As can be seen in the studies listed below, this pattens is seen in the UK Mormons we surveyed. Although the UK samples are not representative, it would be extremely unlikely to obtain a sample of non-Mormons with these characteristics purely by chance - it seems certain that British Mormons also display the unique pattern of fertility seen in the USA; and therefore that Mormonism is the key factor in causing this pattern.
The pattern of Mormon fertility could be explained in a general sense as a combination of two forces: one tending to increase family size, the other tending to limit family size.
1. On the one hand, Mormonism encourages large families, yet
2. On the other hand Mormonism encourages what might briefly be described as 'self-reliance' - in the sense that the ideal Mormon family unit should be able (as a rule) to support itself without assistance from outside the family
(Indeed, the Mormon family ought to aim to generate a surplus to use on church activities such as buildings and mission work - it should also be noted that virtually no money goes on salaries, since Mormon Priests (all adult men in good standing), Bishops and Stake Presidents are all unpaid part-timers - hence indeed they usually subsidise these roles from other jobs.)
3. The combination leads to wealthier Mormon families being the largest, on average, since they can afford to bring up larger numbers of children decently, without using external resources. Presumably poorer Mormons must necessarily limit their family sizes voluntarily by means of contraception.
Reasons for fertility decisions:
The strongest findings across the studies are that large families are desired due to a strong belief in the persistence of family relationships after death. Although this is a factor in many other Christian denominations, this is uniquely emphasised among Mormons - indeed, in theology, the highest unit of salvation for Mormons is not the individual human, but a man and his wife together, whose marriage has been eternally 'sealed' in a Temple.
Devout Mormons therefore, in following LDS church teachings, will wish to have large families.
But family size is of course constrained by the health of husband and wife, and many other things. But there is a strong, albeit not necessarily 'theological' teaching in favour of self-reliance in the Mormon church. This comes out in many ways, such as creating large store of food and other supplies - but also in limiting family size according to economic factors. This constraint does not operate in many other fertile religions, where adherents are happy to accept large amounts of state subsidies (or permanent charitable assistance) in order to sustain large families.
The 2011 and 2012 studies both found a high level of support for the direct mechanism of personal revelation in deciding family size. Being a mystical experience, this is not something that can be operationalized as a psychological variable - suffice to say that Mormons believe that they are given personal and divine guidance on fertility decisions in answer to prayers, often assisted by fasting.