You’ve just learned that your co-worker earns more than you, even though you’ve been with the company longer, and you seem to be better at it than she is. But before storming off to your supervisor’s office to demand a raise, there are reasons why she earns more than you, and you might want to sit down and learn all the reasons why you don’t get the same kind of pay as her.

First, there’s the fact that companies tend to pay new hires a higher salary. As a response to an employee who was disgruntled about this situation, psychologist Art Markman (via Fast Company) said that it’s quite normal for recently hired people to get paid more.

“While there is always the chance that this is related to gender, it may also reflect market forces,” said Markman. He added that since there is a lot of competition for new talent, companies may need to offer higher starting salaries to attract new employees.

The second reason for your co-worker’s higher pay is because she may have negotiated more aggressively for it. According to CNBC, one of the worst mistakes that a new hire can make going into a job is accepting the initial offer without doing some thorough research. Don’t go higher than the industry standard, and have an understanding of why you deserve the money before laying your cards on the table.

Another reason why your co-worker is getting more money is because she may be better dressed than you are. As previously reported by Jobs & Hire, one’s work attire can sometimes be the deciding factor between two candidates when it comes to a promotion, so remember to dress for the job that you want, if you want the promotion with the salary to boot.

Now that you know the reasons why your co-worker is earning more, prepare a presentation as to why you deserve the same amount of money. Visit salary search websites to know how much you should be making, and get your thoughts together before asking for a raise. You may also want to list down everything that you accomplished and contributed to the company to prove that you’re worth it. If you get denied, see if you can get compensated in another way, such as additional paid vacation leaves.