Heres the law for the state of Kentucky, that being said, I always wear my half helmet.
STATUTE:

Title XVI. Motor Vehicles. Chapter 189. Traffic Regulations; Vehicle Equipment and Storage. Section 189.285 Regulations for Operating and Riding On Motorcycles. :
. . . (3) The following persons shall be required to wear protective headgear, in the manner prescribed by the secretary of the Transportation Cabinet, at all times the motorcycles they are riding are in motion on a public highway:
(a) A person under the age of twenty-one (21) years who is operating a motorcycle or who is a passenger on a motorcycle or in a sidecar attachment;
(b) A person who possesses a motorcycle instruction permit and who is operating a motorcycle;
(c) A person who has held a valid motorcycle operator's license, or combination motor vehicle-motorcycle operator's license, for less than one (1) year and who is operating a motorcycle; and
(d) Any person who does not provide proof of health insurance to the county clerk when registering a motorcycle in accordance with Section 2 of this Act.

(4) A motorcycle operator authorized to drive a motorcycle on an instruction permit shall not be authorized to carry passengers.

SECTION 2. A NEW SECTION OF KRS CHAPTER 186 IS CREATED TO READ AS FOLLOWS:

(1) A person over the age of twenty-one (21) years who wishes to operate a motorcycle without wearing protective headgear shall provide proof of health insurance coverage to the county clerk when obtaining or renewing the registration on any motorcycle registered in the person's name. The Transportation Cabinet shall develop, and the county clerk shall provide to the applicant, a decal to affix to the applicant's license plate signifying that the applicant has health insurance and may operate a motorcycle without wearing protective headgear. The applicant shall pay a fee of three dollars ($3) to the county clerk for the decal.

(2) The county clerk shall notify the insurer that upon nonrenewal or cancellation of a health insurance policy for a person under subsection (1) of this section, the insurer shall immediately notify the Department of Vehicle Regulation.

(3) Upon notification to the Department of Vehicle Regulation from an insurance company of the cancellation or nonrenewal of a policy under subsection (2) of this section, the department shall immediately notify the insured. Notification to the insured shall state that the insured's policy is no longer valid and that the insured shall have thirty (30) days to show proof of health insurance to the department or the county clerk. The department shall further inform the insured that if evidence of health insurance coverage is not received within thirty (30) days the department shall suspend the operator's license of the insured for ninety (90) days, or until the person presents proof of health insurance to the department or the county clerk and pays the reinstatement fee required by KRS 186.440 and the relicensing fee required by KRS 186.450.

(If you want to know the reasoning as to why and how Kentucky bikers got themselves in this particular mess, click here.)
FINE:

If you have information about the amount of the fine for violating Kentucky's helmet law, please e-mail it to us. Thanks.
STANDARDS:

Title XVI. Motor Vehicles. Chapter 189. Traffic Regulations; Vehicle Equipment and Storage. Section 189.285 Regulations for Operating and Riding On Motorcycles. :
" . . . (3) The Secretary of the Transportation Cabinet shall by regulation fix minimum standards for approved protective headgear and for approved eye-protective devices, and prescribe the manner in which they shall be used. The secretary shall maintain and cause to be published a list of approved protective headgear and of approved eye-protective devices. The secretary may prescribe by regulation minimum standards for other protective devices and require the use of those devices."