1. "Some of our experimental data comes from studies involving university students, who tend to have limited work experience; future research will attempt to study these situations in a wider variety of organizations."

2. "In addition, some of our effects appear small."

Basically they discount themselves.

Also it's about perception of their EMPLOYERS. Nothing in the study suggests women are less capable or less hireable after two years. It literally says many times in different ways that a long maternity leave could possibly lead to some employers having a negative opinion of a female employee's commitment to their job.

But the problem here is with the employer; why should women be penalized because of this, thus being forced to have shorter maternity leaves?

Being based (at least partially) on university students, these people might not have the same range of commitment say a 30 or 40 year old with a house and established professional network might have. They may genuinely seem uncommitted to their jobs by the nature of their inexperience in the work force (trying out different things, different types of jobs) and place in life. So maybe that really biases the study.

Also it's just ONE study. So one study based on university students, with "small effects", doesn't really prove anything, even if it is published in the Harvard Business Review.

But if I may repeat myself:

If a woman is penalized for taking a long maternity leave because her employer has become prejudiced or biased against her, the problem lies with the employer or some systematic issue in the workforce, and the solution should be addressed there. If it's legal, it should be perfectly fine for her to take the full length of her leave.