Yes, an individual can write his or her own Will, but we don't recommend it for most people. There are many resources, both on-line and in the stores, which offer Will writing services or software. However, the layperson needs to carefully consider whether he or she has the necessary knowledge to accurately set forth what his or her wishes are with respect to how his or her estate will be administered and where the assets will wind up.
It is true that it will be less expensive to write your own Will, or to avail yourself of one of the on-line services. I shouldn't have to tell you, however, that we practicing lawyers spent a lot of years obtaining the education necessary to be an effective estate planner. We also spend a minimum of 15 hours each year obtaining continuing legal education so we can stay abreast of the latest developments in our chosen areas of expertise.
One of the disadvantages of writing your own Will is the problem of being able to verify that you had the required mental capacity to sign the Will. The lawyer who writes your Will will have met with you at least a couple of times and will have asked questions which satisfy the lawyer that you do have the mental capacity required. That lawyer will also document that process and his or her conclusions so that, if the question comes up after your death, there is an adequate record of the testing of your mental capacity. If the lawyer has any doubts about mental capacity, he or she can refer you to a health care professional to help make the determination.
Another disadvantage to writing your own Will is that it will be next to impossible to determine whether or not you signed the Will of your own free will and were under no duress (pressure) to make the Will.
There are several other disadvantages, only one of which is that, unless you are educated about the law (or become educated), you won't know what options are available to you. For example, you won't know whether a simple Will will adequately serve your purposes or if you need a Will that includes trust provisions. The factors we consider when we make recommendations about the type of Will you should have are numerous. One size does not fit all individuals.