Final answer:
The expansion of blue-collar occupations is NOT a significant factor in the increase of women in the labor force, unlike more effective birth control, higher educational levels among women, and increased part-time job opportunities.
Step-by-step explanation:
The question assesses the factors contributing to the increase of women in the labor force. Among the options given, the expansion of blue-collar occupations is NOT a significant factor in this trend. Blue-collar jobs are typically associated with manual labor and have historically been dominated by men. In contrast, the rise in women's workforce participation can be traced back to several other factors.
More effective means of birth control provided women the freedom to choose when to start a family, thereby making it possible to pursue careers. Secondly, the higher educational levels of women have played a crucial role, with more women graduating from college and hence qualifying for a wider range of jobs. Lastly, increased opportunities for part-time employment have allowed women to balance work and personal or family responsibilities more effectively, leading to higher participation rates in the labor market.
It's notable that these trends have affected the natural rate of unemployment differently than cyclical unemployment. However, despite the diversification of the labor force, there hasn't been a long-term upward or downward trend in unemployment rates.