How much lower fertility, less children in the West, is caused by habitual well-being?

From Alexander Solzhenitsyn's Commencement Address, at Harvard University, on June 8, 1978 titled “A World Split Apart”, I here extract “Well-Being": 

“When the modern Western states were being formed, it was proclaimed as a principle that governments are meant to serve man and that man lives in order to be free and pursue happiness. (See, for example, the American Declaration of Independence.) Now at last during past decades technical and social progress has permitted the realization of such aspirations: the welfare state. Every citizen has been granted the desired freedom and material goods in such quantity and of such quality as to guarantee in theory the achievement of happiness, in the debased sense of the word which has come into being during those same decades. (In the process, however, one psychological detail has been overlooked: The constant desire to have still more things and a still better life and the struggle to this end imprint many Western faces with worry and even depression, though it is customary to carefully conceal such feelings. This active and tense competition comes to dominate all human thought and does not in the least open a way to free spiritual development.) The individual’s independence from many types of state pressure has been guaranteed; the majority of the people have been granted well-being to an extent their fathers and grandfathers could not even dream about; it has become possible to raise young people according to these ideals, preparing them for and summoning them toward physical bloom, happiness, the possession of material goods, money, and leisure, toward an almost unlimited freedom in the choice of pleasures. So, who should now renounce all this, why and for the sake of what should one risk one’s precious life in defense of the common good and particularly in the nebulous case when the security of one’s nation must be defended in an as yet distant land?

Even biology tells us that a high degree of habitual well-being is not advantageous to a living organism. Today, well-being in the life of Western society has begun to take off its pernicious mask.”

So, I repeat the question posed: "How much lower fertility in the West is caused by habitual well-being?"

Here I expanded on Solzhenitsyn ending the first quoted paragraph with: 

“So, who should now renounce all this, why and for the sake of what should one risk one’s precious life in defense of the common good and particularly in the nebulous case when the security of one’s nation must be defended in an as yet distant land?”

That helps explains a growing lack of will to defend your nation and the serious difficulties when it comes to recruiting volunteers for the military forces.