7 telltale signs you're in love, according to a scientist who's spent decades studying human relationships

Fisher says that the person you're falling for has begun to take on "special meaning." As one participant in Tennov's study said, "My whole world had been transformed. It had a new center and that center was Marilyn."

Fisher calls this "intrusive thinking."

She writes: "Thoughts of the 'love object' begin to invade your mind. ... You wonder what your beloved would think of the book you are reading, the movie you just saw, or the problem you are facing at the office." Similarly, you mentally review all the time you've spent together.

Many people say these thoughts are distracting to the point that they can't focus fully on work or school.

"Hypomania" is a term for intense energy, and it's associated with the beginnings of romantic love.

Fisher writes that you might experience "trembling, pallor, flushing, a general weakness, overwhelming sensations of awkwardness and stammering." Or, you might find that you're sweating, that your heart is beating wildly, that you've got butterflies in your stomach, or that you can't eat or sleep.

Scientists who study non-human animals use the term "mate guarding" to describe the extreme lengths to which those animals will go to protect their new relationship. Fisher says it applies to humans as well.

You might be terrified of rejection and experience an "intense motivation to win this special person," Fisher writes.

Fisher says participants in Tennov's study described experiencing craving, hope, and uncertainty.

Fisher writes: "If the cherished person gave the slightest positive response, the besotted partner would replay these precious fragments in reverie for days. If he or she rebuffed one's overtures, uncertainty might turn to despair and listlessness."

It's a kind of "separation anxiety," Fisher says.

In fact, any obstacle that stands between you two only serves to increase the romantic passion and craving. Fisher calls this pattern "frustration attraction."

The biggest commonality among Tennov's participants, Fisher writes, was "the feeling of helplessness, the sense that this obsession was irrational, involuntary, unplanned, uncontrollable."

As one survey respondent who was involved in an affair with someone from work put it: "This attraction for Emily is a kind of biological, instinct-like action that is not under voluntary or logical control. ... It directs me."