The natural instinct to survive and to protect family runs strong in human beings. In the 20th century alone, from Jewish families in Nazi-controlled Europe to Soviet dissenters in frigid Siberian gulags, the will to live burns deeply.

So when a major disaster strikes, the instinct to survive will trump all other considerations. Exactly how far people will go depends on the type of disaster, but an understanding of how they will react will help you prepare to protect yourself and your family in these circumstances.

Let's look at two kinds of disasters. The first is when there is a regional catastrophe, like an earthquake, tornado or temporary failure of the power grid. In this case, although there will be hardships in the first 24 hours and likely for weeks thereafter, those affected will believe that FEMA and local aid is working hard to help them.

For example, if there was a moderate earthquake in San Francisco, then FEMA and the California National Guard and 100 other agencies would immediately respond. People would be evacuated from the devastated city and relocated to nearby shelter. They would have a place to stay, dry clothing, and enough food to live in relative comfort until they decide to relocate or move back into the city after rebuilding has occurred.

Although there would be localized violence and crime in this scenario, most people would know that aid was coming and the nation would do everything it could to help.

The situation would be vastly different with the second type of disaster. This is when catastrophe strikes and there's little hope for aid from the government or anyone else.

For example, terrorists or a rogue nation launch an EMP attack, and the nationwide power grid is down long-term. Or, a major solar storm takes out the grid.

In those scenarios, police won't arrive because the pumps don't work and there is no gas. The government can't help because it, too, is without power and gasoline. Millions of people would be desperate, and most of the nation's food would spoil in a matter of days.