Sweden experienced continuously declining fertility rate in early 20th century, and this aroused the population debate. Those who worried most the low fertility rate were the conservatives, while the progressivists holding neo- Malthusian view did not. Alva & Gunnar Myrdal, the spouse with social democratic inclination, published the book The Crisis in Population Problem (Kris i befolkningsfrågan) in 1934 and thereby changed fundamentally the debate terrain. They acknowledged the seriousness of the declining fertility rate, contrary to the conservatives, however, they maintained that it would be possible to raise fertility rate and improve the ‘quality of people’ by progressive family policies. The core idea of their family policy proposals was that society should take the burden of children rearing costs and should support the married working women for holding down job and running household at the same time. Justifications of Alva & Gunnar Myrdal for their family policy ideas included economic logics which can be expressed as ‘economies of scale’, ‘consumer ignorance’, ‘human capital theory’, and ‘internalization of externality’ in modern economics. The philosophical foundations of Alva & Gunnar Myrdal, in terms of their ideological orientation or value orientation, were feminism, socialism, mild nationalism, and utilitarianism. In terms of the approach to social issues, very strong rationalism is found. Especially utilitarianism and rationalism were the hardcore of their philosophy. Even though their approach can be criticized in that they pursued too much the social engineering styled rationality, their contribution should be highly acknowledged if we consider the situation of the Swedish society at that time. Alva & Gunnar Myrdal intended to use the population debate as a good chance for trying progressive social reforms, and their intention was largely realized. Substantial parts of the Swedish welfare state model are debted to the intellectual legacy of Alva & Gunnar Myrdal.